This invention relates to high frequency jointless track circuits for railroads, and while the invention is subject to a wide range of applications, a preferred embodiment of the present invention will be particularly described as applied to a jointless high frequency track circuit system for a stretch of railway track for the detection of occupancy, and for the communication of cab signal controls from the wayside to vehicles passing through the stretch of track.
The present invention is an improvement over currently used jointless high frequency track circuit systems of the general character disclosed in the U.S. patent to W. R. Smith U.S. Pat. No. 3,524,054, issued Aug. 11, 1970. In the system according to this patent, distinctive frequencies are transmitted in adjoining track sections, high frequency transmitters and receivers of these frequencies being coupled to the track rails through impedance bonds having single coil primary windings on which tuned toroid secondary windings are provided for coupling the respective track occupancy detection transmitters and receivers to the track rails. By this arrangement, track circuit receivers are only responsive to the frequencies to which their toroid coils are tuned, and, although the same frequencies are used for more distinct track sections, there is no danger of a receiver being actuated by the same frequency of a distant track section because such frequency is attenuated by intervening shunting primary windings of impedance bonds which are not tuned to the associated frequency. This system of separation of transmitters having the same frequencies is not effective, however, for a cab signal frequency that is transmitted at each location solely for the control of cab signals in vehicles passing over the stretch of railway track. This is because the same cab signal frequency must be used in all track sections for transmission to vehicles passing therethrough, and all of the impedance bonds are tuned to this frequency. Therefore, each impedance bond couples a cab signal code transmitted to the track by a tuned toroid secondary winding which provides for transmission of high frequency cab signal energy equally in both directions from the associated location, although it may be desired to transmit only in one direction toward an approaching vehicle, on stretches of track having only one direction of traffic. Thus, because all impedance bonds are tuned to the cab signal frequency, there can be no attenuation of propagation of the frequency from one adjoining track section to the next as is discussed above where distinctive frequencies are assigned to adjoining track sections for occupancy detection purposes.
To induce enough current in the track rails for satisfactory operation of cab signals according to the system of the Smith patent could be costly because it would require the use of several toroid windings connected in multiple.
An object of the present invention is to provide jointless high frequency track circuit systems for communication of signals to rail vehicles having improvements which substantially obviate one or more of the limitations and disadvantages of the described prior system.
Another object of the present invention is to provide more efficient means for coupling high frequency cab signal energy to the track rails in systems where jointless track circuits are involved.
Other objects, purposes and characteristic features of the present invention will be in part obvious from the accompanying drawings and in part pointed out as the description of the invention progresses.