Railway signalling discussion

Control of block sections

Current and future British signalling (UK except Northern Ireland)

Control of block sections

Unread postby andyhunt873 » Sun Mar 5, 2017 9:40 am

Could someone just clarify that I've got this right please:

If you have a branch line (two lines, one up & one down) and a signal box controlling it's section (block), does that mean that the box controls the down line track coming towards it in one direction, and the up line coming towards it from the other direction?

I know this is probably a very basic question, but that's how I read it.

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Re: Signal boxes - Great Harwood loop line

Unread postby Chris L » Mon Mar 6, 2017 11:20 am

You've confused me slightly by using the term branch line - I thought at first you were talking about a signal box at a junction, but on further reading I've decided that you weren't, you're talking about a box on a "bog-standard" double line bit of railway, where Absolute Block regulations are in force.

Working on that assumption; the signalman at box 'B' (which we'll call the box we're talking about) has to accept a train on the Down line from box 'A' (the box in rear on the Down line) to allow the signalman at 'A' to clear his signals to let it approach him. Likewise, the signalman at 'B' has to accept a train on the Up line from box 'C' (the box in rear on the Up line) to allow the signalman there to clear his signals to let it approach him. Conversely, however, the signalman at 'B' has to offer a train on the Up line to box 'A' (the box in advance on the Up line) , and box 'A' has to accept it, before 'B' can clear his Up Section signal to let it proceed, and he (the signalman at 'B') has to offer a train on the Down line to box 'C' (the box in advance on the Down line), and box 'C' has to accept it, before 'B' can clear his Down Section signal to let it proceed.

So the signalman at 'B' has control of trains approaching him from either direction, insomuch as he can accept or refuse them when offered from the box in rear. However, it is the box in advance of 'B' in either direction which can accept or refuse trains travelling away from him.

John's explained how this is all done on this page: ... ge+2+--%3E (this section of the website is very informative all round and is well worth careful study).

Anyway, I hope that helps.

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Re: Control of block sections

Unread postby Mike Hodgson » Mon Mar 6, 2017 8:31 pm

Just to add to Chris L's explanation, not only does B have to accept a train before the signals may be cleared for a train to approach him, and obviously he won't accept one whilst the previous train is still on its way, but that is reinforced by box A or C not even being allowed to offer him the train until after they have been told (by bell signals) that the first train has vacated the line. That is to say, it would need multiple mistakes to get two trains improperly signalled into the same section, so in answer to your original question, the section is jointly controlled by both signalmen, as they both have to agree that the line is clear. This requirement for co-operation has proved itself as a major safeguard against the risk of human error for well over a century. The block indicators and the train registers act as visual reminders, and it would be most unusual for both signalmen not to notice if something was amiss.

For a deeper understanding of signalling generally I recommend the main Signal Box site (click on the image in the top left hand corner of this web page), which includes the particular page Chris linked to. If anything on that page seems slightly unclear or not self explanatory, another good explanation of the traditional Absolute Block system can be found here This page also describes a few of the electrical controls which effectively prevent signalmen from clearing signals when various circumstances dictate that they should not.

The basic principle of the system is extremely simple, what makes rule books hard to understand are the esoteric exceptions to every rule - the if's and but's, and the abnormal situations such as what to do when trains or signalling equipment break down.
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