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Track Circuit Block

Current and future British signalling (UK except Northern Ireland)

Track Circuit Block

Unread postby Zoe » Thu Apr 29, 2010 9:31 am

Under track circuit block a signal cannot be cleared unless the line up to and including the overlap beyond the next signal is proved to be clear. However looking at some mechanical boxes in TCB areas for example Liskeard there seems to be a strange situation. You first have a berth track circuit as expected for 200 yards before the home but after that there is just another track circuit leading up to the inner home. Would this count as the overlap rather than a specific overlap track circuit?
Last edited by Zoe on Fri Apr 30, 2010 8:03 am, edited 1 time in total.
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Re: TCB and semaphores

Unread postby S&TEngineer » Thu Apr 29, 2010 11:40 am

Yes,

There is no restriction on the maximum length of an overlap track circuit, only on the minimum length it must be. In the case you have quoted at Liskeard this saves the expense of another track circuit for little operational gain.
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Re: TCB and semaphores

Unread postby Zoe » Thu Apr 29, 2010 1:03 pm

I've just had another look at the diagram and what you say makes sense as far as the Down Advanced Starting. The track circuit (I believe actually an axle counter section) after this signal extends all the way to Largin, by that logic you would have to hold a train in Liskeard platform and due to the location of the Down Starting signal not even at the end of the platform as you couldn't prove the line was clear 200 yards beyond 32 until the previous train is at Largin.
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Re: TCB and semaphores

Unread postby Andy Overton » Thu Apr 29, 2010 2:35 pm

The rules cater for this. As the home is semaphore and therefore cannot display a yellow aspect it can't be cleared anyway until the train is almost at a stand at it if the next signal ahead is at danger. In these circumstances you don't need an overlap ahead of the next signal as you are enforcing a restricted approach. If a driver is brought to or almost to a stand at a signal before it is cleared he always understands from that that the line is clear to the next signal only (i.e. no overlap / clearance point).
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Re: TCB and semaphores

Unread postby Zoe » Thu Apr 29, 2010 3:08 pm

Andy Overton wrote:In these circumstances you don't need an overlap ahead of the next signal as you are enforcing a restricted approach.

I thought in TCB areas the interlocking would prevent you clearing a signal if the line is not proved clear up to and including the overlap beyond the next signal. There's no way to detect if a train is actually in motion or not within a circuit so the only way I am thinking this could be done would be to have a timer that would would release the lever after the track circuit had been occupied for a set time.
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Re: TCB and semaphores

Unread postby Mechy » Thu Apr 29, 2010 4:15 pm

Do you have a copy of the diagram in question?
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Re: TCB and semaphores

Unread postby BHornsey » Thu Apr 29, 2010 6:25 pm

Zoe wrote:
Andy Overton wrote:In these circumstances you don't need an overlap ahead of the next signal as you are enforcing a restricted approach.

I thought in TCB areas the interlocking would prevent you clearing a signal if the line is not proved clear up to and including the overlap beyond the next signal. There's no way to detect if a train is actually in motion or not within a circuit so the only way I am thinking this could be done would be to have a timer that would would release the lever after the track circuit had been occupied for a set time.


That is true to to a point, but here I guess you are discussing an older TCB installation when rules were a little different from today.

If you have a semaphore signal, the same instruction regarding clearing it would apply on TCB as it would on AB: If the next signal is at danger, you must bring an approaching train nearly to a stand. The interlocking wouldn't enforce this, the Signalling Regulations do so it becomes YOUR responsibility.

My only experience of semaphores on TCB lines was on the up at Elsenham (North of Bishops Stortford) towards Stansted (Stortford if Stansted was switched out) which had a semaphores (including the starter!) on a TCB line.

To enforce the block (as I remember it) the last track circuit on the up would remain occupied until:
the previous train had passed the overlap of the home signal ahead AND
the home signal had been placed to danger AND
the home signal had to be proven normal or the TC would stay down.

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Re: TCB and semaphores

Unread postby John Webb » Thu Apr 29, 2010 9:06 pm

I understand that at St Albans South when a down slow train was being switched to the up slow platform via a facing cross-over to form a new up service, the semaphore signal allowing this move was 'approach controlled' and the electrical signal lock could not be released until the appropriate TC had been occupied for the set period. I assume this was because there was a permanent stop signal at the far end of the up platform for trains moving in the down direction and this served to remind the driver that the next signal was indeed at danger.
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Re: Track Circuit Block

Unread postby Zoe » Fri Apr 30, 2010 8:05 am

When you clear a multi-aspect colour light signal, does it "read" the aspect of the next signal to know what aspect it should display or does it simply for example read that the next 2 signals are off and so it should display a double yellow?
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Re: Track Circuit Block

Unread postby edwin_m » Fri Apr 30, 2010 11:40 am

In both relay and SSI installations the interlocking determines whether the signal should show a proceed aspect, but the aspect itself is sequenced from the aspect of the next signal.

For relay interlockings the aspects are passed direct from signal to signal by wires on the trackside, the aspect is never brought back to the interlocking and for most auto signals not even the lamp proving is brought back. This is one reason why most panels only show red/green for controlled signals and nothing at all for most autos (exceptions where the signal sometimes needs to be held at danger).

However for SSIs the interlocking works out the aspect of each signal centrally and transmits these out over the data link to the trackside module controlling the signal.
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Re: Track Circuit Block

Unread postby Zoe » Fri Apr 30, 2010 1:54 pm

Jersey_Mike wrote:Because we all know what a great idea it is to put all your eggs in one basket

It will have to be done that way for ERMTS though as lineside signals will be removed and the information trasmitted to the train.
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Re: Track Circuit Block

Unread postby Mechy » Fri Apr 30, 2010 3:11 pm

It seems that the days of having auxiliary panels and geographic interlocking are well and truly over. All it would take is for a disaster at one of these newfangled SCC's and thats it, game over! Then we go straight back to working by hand signallers :wink: . At least with localised operation the railway can keep itself going in the event of a failure. The Victorians had the right idea, in my view things like ERTMS are a step backwards.
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Re: Track Circuit Block

Unread postby Zoe » Fri Apr 30, 2010 3:32 pm

Mechy wrote:It seems that the days of having auxiliary panels and geographic interlocking are well and truly over. All it would take is for a disaster at one of these newfangled SCC's and thats it, game over!

At least they are not taking the NATS approach and putting the entire network in one or two control centres.
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Re: Track Circuit Block

Unread postby Frank » Fri Apr 30, 2010 3:39 pm

Hello,

All it would take is for a disaster at one of these newfangled SCC's and thats it, game over!


ahhh you don`t need a disaster....... :mrgreen:

The Software does it the same way........ :lol:
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Re: TCB and semaphores

Unread postby Andy Overton » Fri Apr 30, 2010 3:51 pm

Zoe wrote:
Andy Overton wrote:In these circumstances you don't need an overlap ahead of the next signal as you are enforcing a restricted approach.

I thought in TCB areas the interlocking would prevent you clearing a signal if the line is not proved clear up to and including the overlap beyond the next signal. There's no way to detect if a train is actually in motion or not within a circuit so the only way I am thinking this could be done would be to have a timer that would would release the lever after the track circuit had been occupied for a set time.

There isn't usually a timer on mechanical signalling but there would be on a route setting installation. With a frame the lever is locked until the berth TC is occupied. The regulations used to be very specific in pointing out that signalmen must themselves ensure that the train had been brought nearly to a stand before pulling off and not take it that just because the signal could be cleared as soon as the berth TC was occupied that it should be cleared. As Brian says, with mechanical signalling the signalman has more responsibility and in effect takes on board many of the functions that can be done by a machine in more modern installations.
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