Single rail DC track circuits

[Archie]

in the case of 'single rail DC track circuits, what is the purpose of the relays from one track circuit to the other abutted to one another?

[PJW]

in the case of 'single rail DC track circuits, what is the purpose of the relays from one track circuit to the other abutted to one another?

Not quite sure what you are meaning......

are you asking when a joint between adjacent track sections would have two track relays adjacent at the IRJ?

are you asking when a track relay of one section would be used to cut the feed to the adjacent track section?

something else?

Can youn re-phrase please

[Archie]

Yeah excuse me, I didn't clearly state that very well, I am asking with regards to your first statement, but an insight into both would be helpful.

Thanks

[PJW]

in the case of 'single rail DC track circuits, what is the purpose of the relays from one track circuit to the other abutted to one another?

Not quite sure what you are meaning......

a) are you asking when a joint between adjacent track sections would have two track relays adjacent at the IRJ?

b) are you asking when a track relay of one section would be used to cut the feed to the adjacent track section?

something else?

Can youn re-phrase please

Other things being equal, there is some advantage of the TR of a track circuit being at the "running on" end- not really when the track is working entirely properly (there is really no volt drop along a dc track, so symmetric) but in the real world when there may be broken bonds over rail joints, possibility of broken rails etc. Hence arranging track circuits relay to feed, relay to feed would be the result. However for most kinds of tracks in most environments not a huge consideration.

Conversely putting two relay ends together can be more economical because if need a lineside cable to call at the location to pick up one TPR linecircuit, then may as well do both; this means that at the other end of the track we can put a pair of feedsets together, so only need a power cable there. Even more benefit when there is SSI; any location needing to pick up an input needs a TFM and that has to be fed by two DLMs and datalinks, so arranging the TPR inputs where we need a TFM for outputs (because there is a signal or a point there) is beneficial. In a station area where there are lots of REBs or locs within the area it makes not much difference; on a long thin bit of railway then it does. Note there are a fixed number of TFMs per datalink, so if use uneconomically by providing a TFM to pick up just one function means more datalinks and more SSI Central Interlockings for a given length of line than if things are arranged that the TFMs are spread out more widely but have a greater percentage of their input capacity used. I think you can see why things are as you described on such schemes.

Other reasons for putting pairs of relays together may be as a consequence of the times that we want to put two feeds together at the adjacent site. See other post re where dissimilar dc tracks can only safely abutt in that configuration.
Similarly if a tunnel is too long for a single track, far better to have a loc with a couple of feedsets in the middle where access is difficult and keep the two track relays at the tunnel portals.

For a track type such as TI21 then a single transmitter can feed two separate tracks in the two directions and be about 175% of the length of a single track, thereby giving further economy of equipment- less space provision, less power, less installation, less maintenance, lmpre reliability since there is less to go wrong.

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re b), this methodology obviously requires the relay of one track to be at the sit of the feed of the next. This is a way of "cascading" two (or indeed more) separate technical track circuits to make a single longer functional one. Hence if the operational need is for a track that is 4km long, but due to ballast resistance constraints the longest that will work reliably is only 1km, then track AA (which the signaller knows about) can be composed of AA/1, AA/2, AA/3., AA/4 (that the technician knows about).

The modern way of achieiving is to run a line circuit picking up each of the TR contacts in a series circuit to create the TPR.

The cheaper way is to save the cable and use the rails themselves. The only contacts of AA/1 TR are used to cut the feed to AA/2 TF etc etc until the contacts of AA/4 TR are used to drive the summation TPR. A bit like repeaters along a transmission line.
When train enters AA/1 then all the sections of the track fall like dominos, but as train progresses each picks up when its own section is clear.
Has some cost advantages and it is impossible to accidentally miss out one of the sections of the track circuit in the summation.
However
1. when there is a failure, the technician has no easy way to narrow down the fault.
2. after P'Way work when S&T need to retest a lot of tracks in a short time, this cascading is an inconvenience.
3. because few of us are used to the arrangement nowadays, when people need to make design alterations, they frequently don't realise the implications of what they are doing and mess it up and can make errors that removes more track circuiting than they intend when making fringe alterations.
4. obviously can't use section AA/4 for approach release of the signal when it is cascaded, since it will drop when train occupies AA/1. Therefore would need to be removed from the cascade with a a permanent feed on its TF and the summation TPR line circuit driven from AA/3 TR but then ALSO needing AA/4 cut in, generally at the interlocking itself before reaching the coil of the relay AA TPR. This is the sort of thing that all too easily can be screwed up; the arrangement is perfectly safe if people aknow what they are doing and are careful, but there is a "trap for the unwary" and big mistakes can be made.......

Hence in the modern environment, tends to be judged to be more hassle than it is worth so we don't put in new and generally remove cascading when doing work in which such an alteration is practicable

[Archie]

Many thanks for that very detailed reply, it was very, very interesting especially with the regards of "cascading" that you mentioned, I was not aware of such practice.