(08-03-2018, 01:01 PM)steak Wrote: I'm stumped.
Im looking at it from the point of track circuits and axle counters.
Axle counter is a discrete track vacancy detection system
Track circuit is a continous track occupancy detection system
I can understand a track circuit positively proving the absence of a train = power supply-track-relay-signal
If the train shunts the track and places signal to danger is this positively proving presence of a train?
I would say a track circuit needs to prove track clear and axle counter needs to positively prove the presence of a train.
Im going round in circles. Been reading as much info as i can but still cant come up with a coherent answer/explanation.
Thoughts would be most welcome!!!!!!
A track circuit attempts to prove the absence of a train; the TR should be energised unless a train 's axle diverts current. Its failure mode is the same as track occupancy, so we cannot really be sure that there is a train there when the TR is down. Have to be careful when using for signal approach release, for keeping a signal off for Last Wheel Replacement, for a permissive move, for releasing an overlap after the signal berth track has been occupied for a time etc.
Whereas we use the axle counter's output similarly, the axle counter does not directly prove absence. The heads detect the presence of a wheel; because they are in pairs then the direction of that wheel's motion can be determined. Hence the evaluator can keep count of the net changes. It has to be told when the section is initially clear and by adding and subtracting it knows when it gets back to the same state again. If it loses power for an instant, then it hasn't got a clue.
A single axle counter head can be used as an "electronic treadle" or "position detector"; indeed London Underground often does. The PDs are used to determine where the front of a train has reached as a form of "belt and braces" before a set of points is regarded as free to be thrown after the dead locking track circuit has cleared. Previously high frequency "overlays" were used instead- the train axle was detected by the relay picking as it completed the circuit in the vicinity of the rail connections (the impedance of the rail prevented a train more than around 10m away being "seen". In the 1980s British Rail also used for implementing approach release to avoid needing to divide the actual track circuit into multiple sections.
Rather unusual but can use a "rail circuit" to detect presence of train by completing electric circuit through the axles. For example the RH&DR used (and probably still does although more recently they use a form of axle counter instead) for initiating level crossings- with rails only 15" apart and alongside the sea with salt water contamination, track circuits not practicable. Relay picked when train within 10m length of track; used much like a treadle contact.
As Dorothy has said, treadles are heavily used for level crossings on NR as presence detectors- for AHBC for example they generally are used to be the back-up means of ensuring that the Strike-in track is dropped by an approaching train. However also used either side of the road in order to determine the direction in which the train has traversed the crossing and get the barriers up again afterwards.
When a train reports its own position relative to the last balise / RFID tag / Norming Point that it encountered, this is certainly a "presence" detection. One of the problems of eliminating lineside track detection is what to do when a train goes non-communicating and how to get the railway working properly again in a safe manner. Even worse if power failure etc. and when trying to resume operations first have to establish how many trains were actually in the area and account for them all.
However as Dorothy says, the question was about SIGNALLING FUNCTIONS, not the form of train detection per se. Actually seems to be a bit more mod 3 than mod 5 in that respect. However I think the things is that for the application you should be aware of the requirement and be aware of the failure modes of the form of train detection selected.
