The present invention relates to computing the length of a railway vehicle or a train of such vehicles.
The concept of replacing conventional railway signalling equipment by train carried, point controlling, and interlocking modules inter-connected by radio or other transmission medium is well known. There are however certain practical difficulties, missing elements, and unnecessary complications in the realization of known systems. One such is the method of determining the location of the tail (or rear) of a train for the purposes of authorizing a following train to proceed or of allowing a route which has been traversed to be released.
In some known systems, train location is determined by an on-board train computer by a combination of reference wayside markers such as transponders, loop ends, loop transpositions or the like together with on-board distance measurement derived from an odometer, tachometer or equivalent device such as a reader of closely spaced track-side marks, plates or loop transpositions or the like. The location thus derived is reported to a wayside control computer which uses the information as the basis for generating movement authorities for transmission to trains with reference also to supervisory controls and fixed interlocking data. For the purpose of route release behind a train and to define the limit of proceed authority for a following train it is necessary for the control computer to deduce the location of the rear of the train. In a known system (British Patent Specification No. 2 189 066)/the control computer does this by reference to the "train consist" (i.e. the overall composition of the train) from which train length can be derived. In practice there are difficulties in ensuring that such information is in a sufficiently vital form for use in safety functions. It would be possible to provide equipment at the rear of the train equivalent to that at the front and to report the location of the rear of the train separately from the front, but this would be complicated and expensive and would double the number of mobile identities to be serviced by the communications system. It is also frequently impractical to locate appropriate equipment at the rear of a train.