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Purpose of Track Circuit Override (TCO)?

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Purpose of Track Circuit Override (TCO)? 05/04/2014 at 00:38 #58409
maxand
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Another dumb question from a dumb newbie. I've read the Brighton manual, read the glossary entry on TCO, read everything about TCO that's not couched in gobbledygook and I'm still no wiser as to WHY I would need a TCO.

Obviously they allow trains out of a siding into an adjacent occupied platform, but the only reason I can think of when this might be desirable would be to allow a train coming out of a siding to JOIN a train standing at the platform, otherwise, you'd end up with a Huddersfield situation.

Why doesn't someone use the word JOIN to describe the purpose of this feature, or am I missing the point completely?

Last edited: 05/04/2014 at 00:40 by maxand
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Purpose of Track Circuit Override (TCO)? 05/04/2014 at 00:44 #58410
Stephen Fulcher
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It is to allow trains to be "called-on".

Some installations in British signalling practice required you to operate a track circuit override function to enable a call-on aspect to be shown to a train at a signal protecting the track circuit to which the override applies.

I suspect that it was there to prevent a signalman accidentally calling a train in on top of one that was already there when he did no intend to do so, but it is not a feature that is generally installed today, although I believe it is still permitted.

In normal running you can ignore it, if memory serves me correct there may not be any trains in the standard timetable that would require you to operate the override.

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Purpose of Track Circuit Override (TCO)? 05/04/2014 at 00:57 #58412
postal
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Just to muddy the waters a bit, there are other sims where you need to operate a TCO in order to allow (or example) a run-round loco to re-join its train and there are TTs (both modern and heritage) for those sims that may require the operation of the TCO. Once again it is data taken from the real life operation so there is no consistency in terms of why or where the TCO is required.
"No question is too stupid, there are just some stupid answers" - Dr. Michael Reece, 12/08/1927 - 03/06/2019. Electrical engineer and inventor
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Purpose of Track Circuit Override (TCO)? 05/04/2014 at 01:37 #58414
maxand
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Stephen Fulcher wrote:
Quote:
Some installations in British signalling practice required you to operate a track circuit override function to enable a call-on aspect to be shown to a train at a signal protecting the track circuit to which the override applies.

I suspect that it was there to prevent a signalman accidentally calling a train in on top of one that was already there when he did no intend to do so, but it is not a feature that is generally installed today, although I believe it is still permitted.
Thanks Stephen, but I'm still confused. If the TCO enables a "call-on aspect" (presumably green) to be shown to an (approaching, shunting) train at a signal protecting the TC, and one condition of the TCO is that the protected TC shows "occupied", then isn't this inviting a collision rather than "prevent a signalman accidentally calling a train in on top of one that was already there"?

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Purpose of Track Circuit Override (TCO)? 05/04/2014 at 02:16 #58415
GeoffM
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It's a kind of "are you sure" feature, though I couldn't tell you why one place would be more likely than another to have one. It is not necessarily for a join operation so that term is not used. Think of it this way: most routes require the track circuits to be clear, so this is overriding that requirement.

The only aspect would be a shunting/calling-on aspect, and nearly always approach controlled at that, so the approaching train's speed would be low.

I've seen new ones in recent years, though I suspect separate entrance/exit controls are preferred instead.

SimSig Boss
Last edited: 05/04/2014 at 02:18 by GeoffM
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Purpose of Track Circuit Override (TCO)? 05/04/2014 at 02:43 #58416
Hooverman
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At Three Bridges we have track circuit override buttons at several stations. At these stations we have had or still have permissive working authorised for both passenger and freight trains (not no be permissively worked at the same time). I'm guessing that since the TCO only apply to the GPLS that it is to prevent the signaller accidentally trying to bring a freight onto a passenger train. Although admittedly Hove of all of our locations that have TCOs, it seems the least likely for that to happen. But going back to when TBASC opened Hove still had an active freight depot receiving domestic coal (No.1 and No.2 up sidings).

We used the ones at Redhill a lot when the intercity/virgin was loco hauled and ran via Guildford and needed to be run round. The ones at Purley were used for the aggregate trains that went to the yard there, before they made the trains longer and now they run round inside the yard. Going back Hove they only really get used for charter trains that need their engines run round.

Getting back to the point about accidentally using them, we you set a route by pudding the entrance button then the exit button and the route sets but you get no change on the signal, then you need to PULL up the TCO button to get the GPLS to clear so I can't see how? Seeing Geoff's answer I do agree with that they function as are you really sure you want to set this route type of function.

Last edited: 05/04/2014 at 02:51 by Hooverman
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Purpose of Track Circuit Override (TCO)? 05/04/2014 at 04:54 #58419
maxand
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Thanks everyone. Maybe someone could incorporate this into a freight timetable for Brighton so we could try it out as a call-on.
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Purpose of Track Circuit Override (TCO)? 05/04/2014 at 05:02 #58420
Hooverman
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I know at least on the Edinburgh scrollies freeware default TT, you used a TCO for the Dunbar run round.
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Purpose of Track Circuit Override (TCO)? 05/04/2014 at 06:47 #58424
Hooverman
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Just for interest, the real TCO buttons at Hove (the three older style buttons mid platform).


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Purpose of Track Circuit Override (TCO)? 05/04/2014 at 07:34 #58427
Josie
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" said:
If the TCO enables a "call-on aspect" (presumably green) to be shown to an (approaching, shunting) train at a signal protecting the TC, and one condition of the TCO is that the protected TC shows "occupied", then isn't this inviting a collision rather than "prevent a signalman accidentally calling a train in on top of one that was already there"?
A call-on aspect isn't a full green. It's given by a subsidiary signal: two white lights on a diagonal underneath the main signal, indicated on a Simsig panel by a white triangle (see RailSigns for examples of what they look like in real life, as well as lots of older versions). The signal is extinguished when not in use, and is then displayed in grey on the panel.

It's used when the signaller wants to send a train into an occupied platform - either to join there or just because two trains are booked to use the same platform at once. The main signal can't be cleared because it has to show red if a train is in the section ahead, so the subsidiary signal gives the driver authority to pass the main signal at danger and proceed as far as he/she can see that it's safe (in other words, not to crash into the other train).

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Purpose of Track Circuit Override (TCO)? 05/04/2014 at 09:56 #58431
maxand
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Thanks Josie for the clearest description so far, also Hooverman for that great pic. As GeoffM said, this would be a lot safer than the usual call-on as shunting approach speeds are low (15 mph).

I'll add a link from TCO in the glossary to this thread.

Last edited: 05/04/2014 at 09:56 by maxand
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Purpose of Track Circuit Override (TCO)? 05/04/2014 at 09:58 #58432
Sacro
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" said:
...this would be a lot safer than the usual call-on as shunting approach speeds are low (15 mph).
A driver would notice no difference, he'd see two whites either way, the only difference is to the signaller and in the interlocking.

Last edited: 05/04/2014 at 09:58 by Sacro
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Purpose of Track Circuit Override (TCO)? 05/04/2014 at 10:15 #58434
Peter Bennet
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" said:
Thanks Josie for the clearest description so far, also Hooverman for that great pic. As GeoffM said, this would be a lot safer than the usual call-on as shunting approach speeds are low (15 mph).

I'll add a link from TCO in the glossary to this thread.
Would you not be better editing the Wiki text rather that having people have the faff of make extra clicks to see the information?

Peter

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Purpose of Track Circuit Override (TCO)? 05/04/2014 at 10:37 #58436
Josie
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Remember that with or without a TCO fitted the main aspect sequence leading to the signal will be working down through yellow to red, so the driver will already be slowing down. The subsidiary signal may not even light until the train is slowly approaching the signal. So TCOs are nothing to do with speeds. ALL they do are require the signaller to operate an additional control (where fitted) before setting a call-on route, in order to force the signaller to think twice about what they're doing.
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Purpose of Track Circuit Override (TCO)? 05/04/2014 at 11:01 #58438
y10g9
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" said:
" said:
Thanks Josie for the clearest description so far, also Hooverman for that great pic. As GeoffM said, this would be a lot safer than the usual call-on as shunting approach speeds are low (15 mph).

I'll add a link from TCO in the glossary to this thread.
Would you not be better editing the Wiki text rather that having people have the faff of make extra clicks to see the information?

Peter
i would second that as you seem to be all about saving making extra clicks everywhere else

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Purpose of Track Circuit Override (TCO)? 05/04/2014 at 11:46 #58440
Hooverman
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Our TCOs at Hove and indeed elsewhere at Three Bridges only work with GPL signals and not the subsidiary signals on the main aspect signals. So the talk of signal sequences from green down to red then the sub coming off with the TCO is irreverent.
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Purpose of Track Circuit Override (TCO)? 05/04/2014 at 12:17 #58443
Steamer
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" said:
" said:
" said:
Thanks Josie for the clearest description so far, also Hooverman for that great pic. As GeoffM said, this would be a lot safer than the usual call-on as shunting approach speeds are low (15 mph).

I'll add a link from TCO in the glossary to this thread.
Would you not be better editing the Wiki text rather that having people have the faff of make extra clicks to see the information?

Peter
i would second that as you seem to be all about saving making extra clicks everywhere else
In any case, reading through this thread is more likely to confuse someone, given that reference was made to call-on aspects being green, which they aren't. Best to extract the relevant and useful information from the thread and summarise it on the Wiki.

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Purpose of Track Circuit Override (TCO)? 05/04/2014 at 13:13 #58444
Danny252
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" said:
As GeoffM said, this would be a lot safer than the usual call-on as shunting approach speeds are low (15 mph).
Eh? Why would it be "a lot safer"? This is practically identical to most call-ons as far as the actual train is concerned.

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Purpose of Track Circuit Override (TCO)? 05/04/2014 at 13:53 #58445
maxand
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Peter Bennet wrote:
Quote:
Would you not be better editing the Wiki text rather that having people have the faff of make extra clicks to see the information?
Many people object to having their utterances paraphrased by others. However, since you lot don't seem to mind, I'm happy to save you all a click or two.

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Purpose of Track Circuit Override (TCO)? 05/04/2014 at 14:32 #58446
Peter Bennet
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" said:
Peter Bennet wrote:
Quote:
Would you not be better editing the Wiki text rather that having people have the faff of make extra clicks to see the information?
Many people object to having their utterances paraphrased by others. However, since you lot don't seem to mind, I'm happy to save you all a click or two.
Rather than objecting it would be preferable for them to correct the WIKI if you don't get it quite right.

Peter

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Purpose of Track Circuit Override (TCO)? 06/04/2014 at 02:48 #58472
maxand
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Before I edit the Wiki I'd like to ask:

In real life, as Josie wrote in post #14:
Quote:
ALL they do are require the signaller to operate an additional control (where fitted) before setting a call-on route, in order to force the signaller to think twice about what they're doing.
The TCO entry in the glossary states:
Quote:
The route is set as normal onto the occupied track, then triggering the override by right clicking on the override button. In most cases, including SimSig, this is represented by a red circle which turns into a full red roundel when pressed.
If the protected track circuit is occupied (e.g., a TC at a platform occupied by a train), which do you do first; set the call-on route or click the override button?

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Purpose of Track Circuit Override (TCO)? 06/04/2014 at 04:15 #58473
Hooverman
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Hello max, you are required to set the route 1st from the GPL signal with the platform track occupied then you use the TCO button.
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Purpose of Track Circuit Override (TCO)? 06/04/2014 at 07:14 #58474
Forest Pines
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" said:

It's used when the signaller wants to send a train into an occupied platform - either to join there or just because two trains are booked to use the same platform at once. The main signal can't be cleared because it has to show red if a train is in the section ahead, so the subsidiary signal gives the driver authority to pass the main signal at danger and proceed as far as he/she can see that it's safe (in other words, not to crash into the other train).
An irrelevant historical note: it wasn't always the case that a main signal couldn't be cleared in such circumstances, certainly on the GWR, but if a subsidiary signal existed it would always be used. The GWR rulebook made it clear that it was possible for permissive working for joining trains to be allowed at locations with only a main signal and no subsidiary, and that at such locations the signalman must clear the signal (rather than give a hand signal).

But that's irrelevant to TCO buttons, because such locations would be unlikely to have TCs anyway!

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Purpose of Track Circuit Override (TCO)? 06/04/2014 at 08:29 #58475
maxand
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Thanks everyone for your contributions, I've edited the Wiki. Hope this is OK.
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Purpose of Track Circuit Override (TCO)? 06/04/2014 at 08:46 #58476
Peter Bennet
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To be honest you seem to have said the same thing several times within the whole explanation. I'll see if I can précis it.


Peter

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