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FEATURE
TRAIN
Location SYSTEMS
CLIVE KESSELL
K
nowing a train’s location is a vital piece of information in the control of train movements - one that has existed almost since
railways were first born.
In the earliest of days, time interval working was used, where trains were dispatched at set times in the hope that the second train would not catch up with the first, but, after a few nasty accidents, something else was needed. Thus, a form of train location system was devised. Advances in technology over the years have led to a number of systems being developed. These can be listed as: » Absolute Block Working - a train’s location is known to be between two adjacent signalboxes, often several miles apart. » Track Circuits - the rail-wheel device that will detect the presence of a train by the wheels providing a short circuit across the rails. Track circuits can vary in distance and can be hundreds of metres in length, so the train location is only known between the track circuit ends. » Axle Counters - a more-reliable replacement for track circuits. However, they also can often count in and out over a long section of track. » Induction Loops - two wires laid out between the rails, with periodic crossover positions, to give reference locations. They constantly transmit information to and from the train, usually associated
Rail Engineer | Issue 183 | April 2020
with Automatic Train Operation (ATO). » Satellite Tracking - technology derived from military and automotive systems, where a train aerial constantly receives the geographical location and displays this either to the driver or is transmitted onwards to a control office. Does not work in tunnels or other covered areas. » Camera Images -a forward-facing camera ‘compares’ the actual image of a train’s position against images held in a reference data base. The resultant position can be transmitted to a control office. » Acoustic Sensing - A train’s vibration
pattern as it progresses its journey is picked up by lineside sensing equipment, usually a fibre optic cable. The resultant change in optical patterns will constantly detect a train’s presence and speed. All of these have strengths and weaknesses. The original requirement of interfacing with the signalling equipment to allow the clearance of signals or the setting of routes is clearly vital in terms of safety, so such devices are invariably SIL4 rated (safety integrity level 4). There is also a need to ensure that a train is complete (that a coupling has not
