US3781543A

US3781543A - Highway crossing protection apparatus

Info

Publication number: US3781543A
Application number: US00251369A
Authority: US
Inventors: C Staples; D Stark
Original assignee: Westinghouse Air Brake Co
Current assignee: Hitachi Rail STS USA Inc
Priority date: 1972-05-08
Filing date: 1972-05-08
Publication date: 1973-12-25
Anticipated expiration: 1990-12-25
Also published as: AU4572572A; AU462081B2

Abstract

Each approach detection track circuit in a multiple track stretch crossed by one or more highways is sequentially connected to a track energy source, including a distance monitor means, by a multiplexing means, shown as a bank of relays operated in repeated cycles by a central process controller. The distance monitor, as it is sequentially connected to each track circuit, provides a distance proportional signal to the central process controller for a train approaching along any track. The process controller registers the detection of the approaching train and determines its continued approach by successive distance measurements received from the same track circuit. When the train is determined to be a predetermined warning time or distance from the crossing, the process controller actuates the highway warning signals. Multiplexing the connections of the track circuits to the distance monitor allows the use of a single set of centralized highway crossing apparatus including the distance monitor and the process controller.

Description

United States Patent Staples et al.

HIGHWAY CROSSING PROTECTION APPARATUS [75] Inventors: Crawford E. Staples, Edgewood;

Donald E. Stark, Penn Hills Township, Allegheny County, all of Pa.

[73] Assignee: Westinghouse Air'Bralre Company,

Swissvale, Pa.

[22] Filed: May 8, 1972 [21] Appl. No.: 251,369

[52] US. Cl 246/130, 246/125, 317/140 [51] Int. Cl B611 1/06 [58] Field of Search 246/125, 128, 130,

[56] References Cited UNITED STATES PATENTS 2,887,571 5/1959 Bettison et al. 246/130 R 3,603,786 9/1971 Peel 246/128 3,619,604 11/1971 Auer etal. 246/122 R 2,424,243 7/1947 Lowell 317/137 3,342,989 9/1967 Dwyer et al 246/122 R Primary Examiner-Robert G Sheridan Assistant Examiner-George H. Libman Attorney-1'1. A. Williamson et al.

[111 3,781,543 1 Dec. 25, 1 973 .detection of the approaching train and determines its continued approach by successive distance measurements received from the same track circuit. When the train is determined to be a predetermined warning time or distance from the crossing, the process controller actuates the highway warning signals. Multiplexing the connections of the track circuits to the distance monitor allows the use of a single set of centralized highway crossing apparatus including the distance monitor and the process controller.

9 Claims, 2 Drawing Figures 1 HIGHWAY CROSSING PROTECTION APPARATUS BACKGROUND This invention pertains to highway crossing protection apparatus. More particularly, our invention relates to protection and/or warning arrangements for com plex rail and highway crossing layouts, for example, multiple tracks with different speeds of approach or plural highway crossings within overlapped warning distances, using a minimum amount of apparatus by multiplexing the detection and control processes to the various approach track stretches.

There is an increasing demand for protecting highway users at grade crossings with railway tracks. This is partly because of public opinion regarding the reduction of highway accidents. However, there does exist a positive need for increased safety, both for highway users and train operation, which requires active crossing warning systems rather than a mere passive or inert wayside sign along the highway. Ideally, of course, a complete grade separation of the highway and railroad tracks is the ultimate safety solution but the physical conditions at many crossings do not permit such improvements. The resulting need, therefore, is to improve the operation and reduce the costof highway crossing protection apparatus arrangements. The reduction of cost is particularly necessary at complex crossing situations where there may be multiple railroad tracks, that is, two or more parallel tracks; where there is a plurality of highways crossing with overlapped warning areas along the single stretch of track; and finally within urban areas where the two situations are frequently combined. A related requirement, of course, is for uniform warning times at any crossing, without regard to varying train speeds and multiple crossings, so that the effectiveness of the warning indication upon the public remains constant and undue periods of waiting will not cause drivers to ignore such signals. One solution for reducing the cost of complex installations is the use of a centralized set of detection and control apparatus that may be time shared over several approach stretches or at the various highways in a plural and complex physical layout. Such centralized apparatus, of course, provides an economicadvantage in reducing the total apparatus requirement and has an operational advantage in that a centralized signal control process may be used having preset warning I conditions.

Accordingly, an object of our invention is an improved highway crossing protection arrangement.

Another object of the invention is a crossing protection arrangement providing improved operation at complex grade crossing layouts.

Still another object of our invention is highway crossing protection apparatus using centralized train detection and warning signal control devices time shared between several approach tracks and/orgrade crossings in a single area.

. It is also an object of our invention to provide highway crossing protectionusing centralized detection and signal control apparatus to provide substantially uniformwarning at each of several adjacent highways intersectinga'single stretch of railroad track.

, Yet another object of the invention is an arrangement at highway crossings for detectingapproaching trains on any one of a plurality of tracks intersecting the highway, using a single set of distance detection apparatus coupled in sequence to each of the several approach track sections.

A further object of our invention is a highway crossing warning arrangement including single train distance detection means connected in sequence by multiplexing means to each of a plurality of approach sections in several parallel tracks intersecting the highway.

It is a still further object of our invention to provide, using centralized train detection and signal control means time shared over various approach track stretches, a relatively uniform warning signal operation at each of several highways intersecting a plurality of parallel tracks over which trains move in each direction across all of the highways.

Other objects, features, and advantages of our invention will become apparent from the following specification when taken in connection with the accompanying drawings and appended claims.

SUMMARY In practicing our invention, a central process controller means including suitable input and output interface apparatus is provided for controlling the warning system for the highway crossing complex. A single distance monitoring device is used to detect the approach of trains toward the crossing or crossings and to provide an output signal proportional to the distance of the train from the crossing. A minimum warning time and- /or distance for each possible approach is predetermined andset into the system, that is, into the process controller. In the principal showing, the distance monitoring device is connected in a repeated sequence to detector track circuits installed in each approach to the crossing along a plurality of parallel railroad tracks. One track circuit at a time is connected to the distance monitor by a multiplexing means, shown as a bank of relays controlled by the process controller. Each track circuit is operable for detecting a train when energized through the distance monitor, and also to provide the distance monitor with a signal for determining the distance to the approaching train. The train distance signal is fed in digital form into the process controller which compares successive distance measurements to determine the speed of the approaching train and thus the time prior to its arrival at the crossing. When the determined arrival time of a train is equal to the predetermined minimum warning time for the corresponding approach, the process controller actuates the warning signal means to indicate to highway users that a train is approaching within the preset distance or time. If a plurality of closely adjacent crossings are positioned along the stretch of track soas to have overlapped approach sections or distances, a minimum time for each crossing is set into the controller so that, with the single approach detection track circuit, the various warning signals at each crossing may be individually actuated at the proper time during the passage of the train. The signal control process may also include a detection of trains entering the stretch from a spur track within the approaches to determine which and when various crossing signals are actuated.

DETAILED DESCRIPTION- We shall now describe the arrangement of our invention in more specific detail, prior to pointing out the novel features thereof in the appended claims, referring from time to time to the accompanying drawings in which:

FIG. 1 is a diagrammatic circuit representation of a crossing warning system for a multiple track crossing by a single highway embodying the arrangement of our invention. 4

FIG. 2 is a schematic illustration of a multi-crossing installation along a stretch of track, to which the invention is applied using a different type of detector track circuit.

I In each of the figures, similar items are designated by similar reference characters.

Referring to FIG. 1, three parallel railroad tracks, each shown by a double line representation, are illustrated as intersected by a highway H, which is also shown by a double line symbol. A warning signal device is located along the highway on each side of the railroad tracks to' warn highway traffic moving in each direction of the approach of a train. These signal devices, designated as G1 and G2, are shown by conventional symbols as well-known flashing light apparatus. It is understood, of course, that other types of warning signals are available, forexample, automatic gates in addition to the flashing lights may be provided, and the-use of any such alternate or additional warning apparatus is included in the arrangement of our invention; The necessary and proper operating controls for the warning signals are supplied through an-input-output interface apparatus from a central process controller, which will be discussed later.

Since trains may move in either direction on each of the three tracks, the detection of trains approaching from either side of the highway must be accomplished. For this purpose, a detector track section in each direction along each track is provided, separated by the insulated joints J shown in each rail immediately to the right of highway H. Each approach track section is terminated by a shunt S connected between the rails at the distant end of the section, the points of connection of the shunts being predetermined in accordance with allowable train speeds and the desired detection times. Each track section is designated by the reference T with a prefix in accordance with the track number, in

sequence, from the top, and an indication as to whether the section extends to the west (left) or to the east (right) of the highway. For example, in the upper or track No. l, the two approach'detector sections are lWT, extending to the left, and lET, extending to the right.

Each track section is provided with a track circuit for the detection of approaching trains. Since energy, as will be described, is supplied from the highway end and with a permanent shunt across the distantend, each track circuit is of the type which permits the measurement of the distance to the permanent shunt or to an intervening shunt provided by train wheels and axles. Each track circuitis normally deenergized and energy is supplied periodically through a distance monitor device, shown as a conventional block so labeled. This energy is preferably an alternating current of a preselected frequency which will permit an indication of the distance to the rail shunt. The distance monitor device is shown by a conventional block since the detailed circuitry thereof is not a particular part of our invention and may be of any known type. For example, one such device is shown in Letters Patent of the U.S. Pat. No. 3,342,989, issued Sept. 19, 1967, to E. C. Dwyer and B. Mishelevich, for a Track Fullness System. Another type of such distance monitor devices which may be adapted to use in the presently disclosed arrangement is shown in Letters Patent of the U.S. Pat. No. 3,155,350, issued Nov. 3, 1964, to R. D. Campbell, also for a Track Fullness System. Either of these disclosed distance measuring means or any equivalent apparatus may be adapted for use in an arrangement embodying our invention.

Although direct rail connections may be used, energy is normally coupled to therails of each section through a track transformer. Herein each is designated by the reference T with a combined suffix including the track number and a letter W orE, designating the associated west or east track section, respectively. For example, for track sections IWT and lET, the track transformers are designated by the references T1W and Tl E, respectively. The secondary of each transformer is connected across the rails, the two secondaries in each track being insulated from each other by the joints J. The primary of each transformer is periodically connected to the source of alternating current energy through the distance monitor device.

Thetrack circuits may be considered to be normally deenergized since they are only periodically supplied with energy from the distance monitor apparatus which serves as a constant current source. The sequence of application of energy to the various track circuits, that is, their connection to the distance monitor apparatus, is controlled by a multiplexer means, shown specifically as a bank of relays A, B, and C. These relays,'in their sequencing operation, are illustrated as being controlled through the interface circuitry by the process controller, which is to be discussed later. The time sharing or multiplexing connection sequence for supplying energy from the distance monitor to the track circuits is actually performed by the matrix of contacts of relays A, B, and C. The contact matrix connections, as shown, are for binary operation of the relays with relay A representing the least significant binary digit, and relays B and C being of increasing significance in that order. Since only six track sections or track circuits are specifically illustrated, two of the possible eight code combinations of the three relays in binary operation are unused.

Thefirst multiplex circuit connection used is that with relay A alone energized, designating the binary code 1. Under these conditions, energy is supplied to the track circuit of section 2ET from terminal BX of a suitable alternating current source, which may be a commercial frequency source, through the distance monitor and over front contact a of relay A, back contact a of relay B, back contact b of relay C, and the primary of transformer T2E to terminal NX of the alternating current source. During the immediately following relay code combination, relay B alone is picked up, signifying a binary 2. The completed circuit now includes back contact a of relay A, front contact b of relay 8, back contact 0 of relay C, and the primary of transformer TIE, so that the track circuit of section lET is supplied with energy. During the next relay combination representing binary 3, relays A and B are both picked up and the circuit over front contacts a of these two relays and back contact a of relay C supplies energy to the primary of transformer T3E for the track circuit of section 3ET. The binary 4 combination, relay C alone picked up, is not used in this specific showing. During the following relay combinations, the track circuits for sections ZWT, lWT, and 3WT, in that sequence, are energized, as may be traced by reference to the drawing. The binary 0 combination of all relays released is also not used. However, the possible circuit connections shown from back contact b of relay B by dotted lines, including front and back contacts d of relay C, could provide energy to two additional track circuits during the herein non-used relay combinations. The cycle of relay operation in the multiplexing arrangement normally immediately repeats and such continuous repeating of the cycle occurs under control of the process controller to periodically provide energy to detect train approach through each of the track sections. Obviously, other relay operational sequences can be used and the contact matrix connections revised to accommodate such operations. As each track circuit is energized during the sequential operations, the distance monitor device checks for train approach detection and develops a distance measurement signal to the permanent shunt S or to the shunt resulting from the entrance of a train into the section.

Although shown as a bank of relays, obviously other switching apparatus may also be used as the multiplexing means. For example, transistors or controlled rectifiers can replace the relay contacts and the gating of such devices in selected sequence controlled by signals from the process controller. Further, the multiplexing means, relay or solid state, can be designed, if desired, to be free running or self operating through at least the normal predetermined sequence without control signals from the process controller. This latter device could then exercise specific control to vary the normal sequence if extra scanningof specific track circuits is desirable, e. g., one occupied by a train. If free running, the multiplexer would of necessity indicate to the process controller the step or condition in the normal sequence to which it had advanced.

The process controller means, shown in the lower left of FIG. 1, is, the heart of the control for the crossing system, determining whenthe warning signals are actuated to provide sufficient but not excessive warning time to highway users. The degree of sophistication of uniform operating time desired in any particular installation fixes the requirements for the process controller operation. This device is herein shown by a conventional block since it may be any one of different types comprising known circuit elements. The operation desired can be provided by any of the types known and it is the operation only, and not the specific circuit details which are included as part of our invention. This process controller may be either a hard wiredor a stored program apparatus. In either case, the operating functions provided are the same. This device sorts the incoming data, particularly the train detection and distance information, and registers it on a per track circuit basis. It calculates the distance to the approaching train and provides for scaling of the individual approaches where their characteristics differ. By comparison of the successive distance calculations for a track, the veloc ity of an approaching train and its direction of travel may be determined. Once the distance, velocity, and/or way traffic. The process controller, of course, provides the actuating controls for the warning signal devices, the control being of a nature to match the type of signal provided. The process controller also controls the sequential track circuit scanning cycle by controlling the operation of the multiplexer, here relays A, B, and C. In installations where several highway crossings exist in relatively close relationship along the same stretch of tracks, the functions of the controller can provide implementation of the different highway warnings, without having separate track circuits, by being programmed with the required sequencing and timing for actuating and clearing the successive signals during the passage of a particular train.

The distance proportional signal or indication provided by the distance monitor for any particular track circuit is digitized by the analog to digital converter and then applied through the interface to the process controller. The digital output may be of binary form but is not limited to this type of digitizing. The resolution of the converter is such as to provide measurements of the precision necessary to establish the location of the approaching rail traffic to the accuracy required by the overall system requirements. The analog to digital converter is shown by a conventional block since the specities of the circuitry depend upon the type of distance monitor used and the required accuracy of the conversion for input to the process controller. Since such converters are well known in the art, the selection of the specific type required may be made during the design engineering for a particular highway crossing installation.

The input and output interface apparatus, also shown by a conventional block, provides the interconnection between the distance monitor and associated analog to digital converter outputs and the process controller and also couples the outputs from the process controller to control the multiplexing relays and the various highway signals. The specifics will depend upon the requirements of the individual installation, the type of process controller used, and the characteristics of the input and the output signals developed. The input portion of the interface accepts the digital data from the A/D converter and passes it to the process controller. The output portion of the interface provides for control of the multiplexer relays and for the control of the other external equipment, such as the warning signals which may include crossing gates, flashing lights, and bells. The energy levels and power handling capabilities of the various circuits are proportioned to accommodate the need of the devices they control. In addition, the interface may also contain supplementary apparatus, such as times and flashers, for control of the warning signals. As indicated, the size and sophistication of the interface circuitry and apparatus will depend upon the type of process controller selected and the signals required for operation of the various external apparatus. FIG. 2 shows a single track stretch intersected by a plurality of highways H1, H2, and H3, which are in relatively close proximity to each other. Although only a single track is shown, obviously the several highways could intersect parallel tracks, such as are shown in FIG. I. It has already been mentioned that the process controller embodied in the arrangement of our invention can be wired or programmed, depending upon its type, to handle a situation such as shown in FIG. 2. Under these conditions, the proper actuation of the warning signal at each highway, in order to provide sufficient warning of proper duration to the highway users as the train passes along the stretch, can be accomplished.

Also shown in FIG. 2 is an alternate type of track circuit which may be used when it is desired to avoid any interference between train detection for the highway crossing warnings and the regular track circuit detection of the railroad signal system controlling the movement of trains. Although shown inthe specific circumstances illustrated in FIG. 2, such a track circuit may actually be substituted for the pair of track circuits in each one of the parallel tracks shown in FIG. 1. Specifically, each track circuit shown in FIG. 2 is of a center fed type and the connections from the secondary winding of the track transformer T1" span the width of the associated highway to provide positive detection over the crossing without requiring multiple track feeds or other special handling. A tuned shunt TS is shown at each distant end of the approach track sections since it is intended that the energy source will supply audio or higher frequency current for overlay track circuit operation, which is well known in the art. Such overlay track circuits include no insulated joints and thus do not interfere with the regular train detection track circuits of the railroad signal system. The capacitor shown in series with the secondary of each track transformer TT, in conjunction with the inductance of the transformer winding, provides a high impedance to signals of other frequencies which may be used for train detection in the train signal system, and thus preserves the integrity of the train detection track circuits otherwise in use. Dependingupon the existing circumstances, the energy source for each track circuit may be coupled or connected to the rails by other known arrangements.

Three distinct track circuits are shown, one for each highway crossing. Each track circuit uses the same pair 4 of distant shunts TS but applies track current at the associated crossing. The secondary of each transformer TI is connected to the rails to span the associated highway for the purpose previously mentioned. Specifically, the secondary connections of transformer -'I'Il span highway 1-11; of transformer TT2, highway H2; and of transformer 'I'I3, highway H3. The primary windings of transformers 'I'Il, TT2, and TT3 are supplied with energy in sequence from the distance monitor, with the sequence controlled by a multiplexingmeans such as the relays A, B, and C of FIG. 1. Thus each track circuit is energized at a different time and no interference results. Obviously, each overlay track circuit shown in FIG. 2 will detect the presence of the train within the limits defined by the distant tuned shunts TS and, in conjunction with the distance monitor means which supplies track energy in sequence, provides a distance proportional signal to the process controller for the installation. This latter apparatus, as previously described, then functions to calculate the distance to the approaching train from any one of the highway crossings shown and actuates the corresponding warning signals at the proper time during the movement of the train through the stretch of track shown.

It will be understood by those skilled in the artthat a failure in either type of track circuit, under which the track circuit becomes open due to a break in a rail or interruption of the connection of the shunt across the rails, will result in a much higher distance measurement from the distance monitor means. The process control ler may be programmed or wired to respond to such a fault condition to actuate the signals since the much higher distance indication will indicate a fault which may preclude the detection of an approaching train.

The system of our invention thus provides an economical arrangement for providing highway crossing warning signals for any highway crossing installation and particularly one which is of a complex nature due to the presence of a plurality of parallel tracks and/or several closely spaced highway crossings. The central process control means and the multiplex use of the train detection and distance monitoring apparatus reduces the total apparatus requirement for the whole installation. At the same time, the efficiency of the centralized process controller to determine the distance and arrival time of approaching trains, and to compare these with the. preset warning distances and times, allows the proper operation of the warning signals at each of several crossings and for trains approaching over any one of the tracks. The finalresult then is an efficient, effective, and economical highway crossing protection apparatus.

Although we have herein shown and described but two forms of highway crossing protection apparatus embodying our inventive concept, various changes and modifications therein within the scope of the appended claims may be made without departing from the spirit and scope of our invention.

Having thus described our invention, what we claim 1. A highway crossing protection arrangement for a highway intersecting multiple parallel railroad tracks, comprising in combination,

a. a normally deenergized separate track circuit for each direction of approach to said highway along each parallel track, capable when energized of detecting the presence of a train within approach warning limits,

b. a warning signal means along said highway operable when actuated for displaying a signal warning highway users of the approach of a train,

c. distance monitor means for at times individually supplying energy to a particular track circuit and responsive to the detection of a train therein for determining the distance of the approaching train from said highway,

d. a mutliplexing means operable for sequentially coupling said distance monitor means individually to eachseparate track circuit, and

e. a process controller means coupled for receiving .the approaching train distance information from said distance monitor means and responsive thereto for actuating said warning signal means to display the warning signal when the train is a prose lected minimum time period from said crossing.

2. A protection arrangement as defined in claim 1 in which,

a. energy supplied by said distance monitor means is an alternating current of preselected frequency, and

b. each track circuit includes,

1 a transformer for coupling said distance monitor means to the rails of the corresponding approach track section, and

2. a shunt connected across the rails at the end of the corresponding section distant from the cross- 3. A protection arrangement as defined in claim 2 in which,

a. the separate track circuits for each direction of approach along each parallel track are paired as a center fed track circuit, coupled to the rails by a single transformer to form also a positive detector section including the actual crossing,

b. each shunt at the distant end .of each approach section is tuned to said preselected frequency, and

c. the rail connections of each coupling transformer include a series capacitor selected to inhibit the shunting of any other superposed track circuit of different frequency.

4. A protection arrangement as defined in claim 2 in which said multiplexing means includes,

a. a bank of multiplexer relays controlled by said process controller means for operating through a predetermined sequence of conditions in repeated cycles, and

b. a contact matrix controlled by said relay bank for completing a different circuit path through said matrix for each sequential condition of said relays,

0. each said circuit path connected for supplying energy from said distance monitor means to a different track circuit to detect an approaching train in the corresponding section.

5. A protection arrangement as defined in claim 4 which further includes,

a. interface apparatus coupling said process controller means to said relay bank and to said warning signal means, and

b. analog to digital converter means connected for receiving train distance signals from said distance monitor means and operable in response thereto for converting each signal from analog to digital form,

1. said converter means being coupled through said interface apparatus for supplying each digital distance signal to said process controller'means.

6. A highway crossing protection arrangement for a plurality of highways intersecting in relatively close proximity a railroad track, comprising in combination,

a. a warning signal means along each highway opera ble when actuated for displaying a signal warning highway users of the approach of a train,

b. a normally deener gized detector track circuit for each crossing capable when energized of detecting a train approaching that crossing in either direction within limits of that track circuit,

c. a shunt connected across the rails at a location along said track a selected distance in each direction away from all said plurality of crossings for defining the distant limits of each track circuit,

d. distance monitor means for at times supplying en-' ergy to each track circuit and responsive to the detection of a train therein for determining the distance of that train from the associated highway crossing,

e. a process controller means coupled for receiving train distance information from said distance monitor means and responsive thereto for actuating a selected warning signal means to display the warning signal when an approaching train is a predetermined time interval from the corresponding crossing, and

f. a multiplexing means operable for sequentially coupling said distance monitor means to each detector track circuit.

7. A protection arrangement as defined in claim 6 in which,

a. energy supplied by said distance monitor means is an alternating current of preselected frequency,

b. each shunt at the distant ends of said track circuits is tuned to said preselected frequency, and

c. each track circuit further includes,

1. a transformer for coupling said distance monitor means to the rails in a manner to form also a positive detector section spanning the corresponding highway crossing, and

2. a capacitor connected in series in the rail connections of said coupling transformer to inhibit the shunting of any other superposed track circuit of different frequency.

8. A protection arrangement as defined in claim 7 in which said multiplexing means comprises,

a. a bank of multiplexer relays controlled by said pro- .cess controller means for operating through a predetermined sequence of conditions in repeated cycles, and

c. each said circuit path connected for supplying energy from said distance monitor means to a different track circuit to detect a train.

9. A protection arrangement as defined in claim 8 in which,

a. said process controller means is a digital data process controller, and which further includes,

b. interface apparatus coupling the process controller output to said multiplex relay bank and to said warning device, and

c. analog to digital converter means coupling said distance monitor means through said interface apparatus to said process controller and operable for changing the distance signal output of said distance monitor means from analog to digital form.

=k l t l= i=

Claims

1. A highway crossing protection arrangement for a highway intersecting multiple parallel railroad tracks, comprising in combination, a. a normally deenergized separate track circuit for each direction of approach to said highway along each parallel track, capable when energized of detecting the presence of a trAin within approach warning limits, b. a warning signal means along said highway operable when actuated for displaying a signal warning highway users of the approach of a train, c. distance monitor means for at times individually supplying energy to a particular track circuit and responsive to the detection of a train therein for determining the distance of the approaching train from said highway, d. a mutliplexing means operable for sequentially coupling said distance monitor means individually to each separate track circuit, and e. a process controller means coupled for receiving the approaching train distance information from said distance monitor means and responsive thereto for actuating said warning signal means to display the warning signal when the train is a preselected minimum time period from said crossing.

2. a shunt connected across the rails at the end of the corresponding section distant from the crossing.

2. A protection arrangement as defined in claim 1 in which, a. energy supplied by said distance monitor means is an alternating current of preselected frequency, and b. each track circuit includes,

3. A protection arrangement as defined in claim 2 in which, a. the separate track circuits for each direction of approach along each parallel track are paired as a center fed track circuit, coupled to the rails by a single transformer to form also a positive detector section including the actual crossing, b. each shunt at the distant end of each approach section is tuned to said preselected frequency, and c. the rail connections of each coupling transformer include a series capacitor selected to inhibit the shunting of any other superposed track circuit of different frequency.

4. A protection arrangement as defined in claim 2 in which said multiplexing means includes, a. a bank of multiplexer relays controlled by said process controller means for operating through a predetermined sequence of conditions in repeated cycles, and b. a contact matrix controlled by said relay bank for completing a different circuit path through said matrix for each sequential condition of said relays, c. each said circuit path connected for supplying energy from said distance monitor means to a different track circuit to detect an approaching train in the corresponding section.

5. A protection arrangement as defined in claim 4 which further includes, a. interface apparatus coupling said process controller means to said relay bank and to said warning signal means, and b. analog to digital converter means connected for receiving train distance signals from said distance monitor means and operable in response thereto for converting each signal from analog to digital form,

6. A highway crossing protection arrangement for a plurality of highways intersecting in relatively close proximity a railroad track, comprising in combination, a. a warning signal means along each highway operable when actuated for displaying a signal warning highway users of the approach of a train, b. a normally deenergized detector track circuit for each crossing capable when energized of detecting a train approaching that crossing in either direction within limits of that track circuit, c. a shunt connected across the rails at a location along said track a selected distance in each direction away from all said plurality of crossings for defining the distant limits of each track circuit, d. distance monitor means for at times supplying energy to each track circuit and responsive to the detection of a train therein for determining the distance of that train from the associated highway crossing, e. a process cOntroller means coupled for receiving train distance information from said distance monitor means and responsive thereto for actuating a selected warning signal means to display the warning signal when an approaching train is a predetermined time interval from the corresponding crossing, and f. a multiplexing means operable for sequentially coupling said distance monitor means to each detector track circuit.

7. A protection arrangement as defined in claim 6 in which, a. energy supplied by said distance monitor means is an alternating current of preselected frequency, b. each shunt at the distant ends of said track circuits is tuned to said preselected frequency, and c. each track circuit further includes,

8. A protection arrangement as defined in claim 7 in which said multiplexing means comprises, a. a bank of multiplexer relays controlled by said process controller means for operating through a predetermined sequence of conditions in repeated cycles, and b. a contact matrix controlled by said relay bank for completing a different circuit path through said matrix for each sequential condition of said relays, c. each said circuit path connected for supplying energy from said distance monitor means to a different track circuit to detect a train.

9. A protection arrangement as defined in claim 8 in which, a. said process controller means is a digital data process controller, and which further includes, b. interface apparatus coupling the process controller output to said multiplex relay bank and to said warning device, and c. analog to digital converter means coupling said distance monitor means through said interface apparatus to said process controller and operable for changing the distance signal output of said distance monitor means from analog to digital form.