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TCB and Semaphore Signals

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TCB and Semaphore Signals

Unread postby Jamesthenovice » Thu Jul 6, 2017 11:02 am


Just got a question with TCB and the use of Semaphore signals. So with Absolute Block, Signal Boxes communicate via block instruments to acknowledge/ request permission for a train to move forward to the next block section.

How does this happen with track circuit block and semaphore signals? In terms of the signaller knowing that a train is approaching etc. Do all boxes on TBC with semaphore signals have panels which indicate the track circuit that a train is occupying?

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Re: TCB and Semaphore Signals

Unread postby Peter Gibbons » Thu Jul 6, 2017 9:17 pm

If I understand you correctly, then yes all approach and departure track circuits will be illuminated when occupied, normally by means of an illuminated track diagram.

Littlehampton for example, a terminal stations has four platforms which each have a semaphore signal. However the track layout is fully track circuited so therefore the signaller does not need the permission of the signal box ahead in TCB areas. Often there will a buzzer, or other audible advice that will alert the signaller in the box ahead of the approach of a train into his/her area, at least that's my experience.
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Re: TCB and Semaphore Signals

Unread postby StevieG » Thu Jul 6, 2017 11:51 pm

In case of any further help Jamesthenovice, regarding allowing trains to run forward from the area controlled by 'your' box - let's call it 'Box B' [as in the traditional and oft-used method of describing methods of working by referring to what happens between three boxes along a route by labelling them A, B and C], the track diagram (or panel / VDU) at 'B' will include showing the 'Clear'/'Occupied' state of all the track circuits along the line B->C, from Box B's last controlled signal up to the next stop signal (irrespective of whether it is an automatic signal or one controlled from Box C), plus an 'Overlap' beyond it [a safety precaution against accidental overrun of that next signal if at Danger (by having proved a further short section of track to also be 'Clear', frequently about 200 metres, but length can vary) ].

As soon as a train is ready to leave, or approach and pass, B, it can normally be allowed to do so provided that all those track circuits are showing 'Clear'.

Then if we look at the same train as leaving B and proceeding towards C [ the same would apply for a train from A proceeding towards you at B ], the track diagram (or panel / VDU) at C should show the 'Clear'/'Occupied' state of all the track circuits along the line B->C, commencing at Box B's last controlled signal.
And, as has been said, the signaller at C needs an alert of it coming, as each such signaller needs to know the location of each train in his/her area of control/supervision including when and where further trains enter that area, and it can also be necessary in order to give time to ensure that signals for trains are cleared when appropriate and safe to do so before causing delay.
This alert could very basically be, also as said, by a buzzer but there needs to additionally be further information of the train's type / identity. This can be by 'block' bell employing the usual 'Is Line Clear for?' bell-code of Absolute Block (AB), (but here used just as a 'Train Description' (TD) to inform of the train type rather than ask permission for it to come), or the TD can be far more commonly received by some visual means - these days most usually by display of the 4-character train identity code as provided in Working TimeTables or pre-supplied supplementary information thereto.

You probably realise that the above means that the state(s) of a certain stretch of the track-circuiting of the B->C line are therefore displayed at both B and C.

I would add that the foregoing applies irrespective of whether none, any or all of the running signals are semaphores.
Semaphores governing entry into (/ exit from) TCB areas was at one time not at all uncommon, especially where the TCB sections were short and/or existed in areas where most box-box sections were AB-worked and boxes were only a few miles apart.
These days I would think that the number of semaphore/TCB instances must have become fairly small, and many of which probably exist where traditional signalling fringes with extensive resignalled areas working TCB and associated with large control centres, though many such fringe boxes have had their semaphores also changed to be colour-lights at the time of becoming an AB / TCB 'fringe'.

One important feature about semaphores in the context about which you enquired is that stop signals of colour-light type protecting entry into a TCB section, are automatically placed to Danger by a train occupying track circuit(s) beyond them.
So the signalling equipment where there are semaphores, unless they are power-operated and are arranged to auto-revert to Danger or are equipped with some other equivalent means, must incorporate means of not allowing a train to approach a semaphore which is still 'Off' at clear after the previous train has passed, until it has been put back to Danger.

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Re: TCB and Semaphore Signals

Unread postby alancolq » Sun Jul 23, 2017 10:46 pm

Hi James,

Just to add some notes to StevieG's very comprehensive reply, I concur that block bells were often used to describe trains between manual signal boxes with semaphores where the mode of signalling was TCB. An example from the Edinburgh area until closure in 1978 was the section between Saughton Junction and Queensferry Junction on the Edinburgh - Glasgow main line. Each SB had manually operated semaphores, with the block section broken by a set of "autos" (colour light, automatically operated home and distant signals). Whenever the illuminated diagram showed that the track circuits between the semaphore starting signal and the overlap at the auto home signal in advance had cleared, the semaphore starting signal lever would be electrically released (an automatically triggered equivalent of getting a line clear from the box in advance).

So, at Saughton Junction for example, whenever a train was accepted on the Down South line from Haymarket West Junction, the semaphore home and distant signals would be cleared right away (or as soon as the overlap track circuit at the down auto had cleared). As the train passed the box, the signalman would "bell it on" to Queensferry Junction in the usual manner - call attention, followed by the appropriate description code after call attention was acknowledged. The signalmen at either end always knew that the train was passing the box in rear whenever it was "belled on". The time would be recorded in the TR book in the "time description sent" or "time description received" column as appropriate.

At Saughton Jct, the block bell to Queensferry Junction was actually a BR-standard 3-position block instrument. This was because Queensferry junction was able to switch out, in which case Saughton Jct would work to Bathgate Junction by Absolute Block - the intermediate auto signals would be permanently "off" (green aspects), and the semaphore starters locked by the block.

Note that at some locations it could be the case that TCB was in operation on one line while AB was in use on the other - e.g. between Winchburgh Junction and Linlithgow with TCB on the Down Main and AB on the Up.
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Re: TCB and Semaphore Signals

Unread postby Chris Rideout » Thu Jul 27, 2017 10:21 pm

Jamesthenovice wrote:Hi,

How does this happen with track circuit block and semaphore signals? In terms of the signaller knowing that a train is approaching etc. Do all boxes on TBC with semaphore signals have panels which indicate the track circuit that a train is occupying?


At Southampton Central after 1966, the Up and Down Mains were TCB on the Northam Junction side. The last semaphore signal was no. 11 (Up Main Advanced Starting) at the tunnel mouth. That had an auto semaphore distant under it and the next signal was a 3 aspect colour light just after the tunnel. Originally, there was a clock describer for down trains but I was there when the up (transmitting) and down (receiving) magazine describers were introduced around 1970. There was just a block bell and there were no TES or TOS bell codes. Trains were described as they passed the box or left the station.
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