Signalling terminology: 'Blocking Back'?

[Smudger105e]

I am an experienced Railwayman, but my specialism is Engineering of T&CS.

Reading the Quintinshill thread, there is some terminology which I don't understand.

Can someone explain what is "blocking back", when is it done and how/why?
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I am an experienced Railwayman, but my specialism is Engineering of T&RS.

Reading the Quintinshill thread, there is some terminology which I don't understand.

Can someone explain what is "blocking back", when is it done and how/why?

Damned auto correct!

 

[sbt]

Blocking Back: "Term used in Absolute Block and Electric Token Block, when a train or
Shunting Movement is to be allowed to occupy the line within the Clearing
Point or on the Approach To the Home Signal."

From the Railway Group Guidence Note: Signalling Terms, April 2004 version

http://www.rgsonline.co.uk/Railway_... Signalling/Guidance Notes/GKGN0802 Iss 1.pdf

 

[Smudger105e]

Thanks. I understand a bit better now although I can't open the link on my phone.

What action would the preceding 'box need to take on receiving the bell signal, and how is the notification cancelled?

 

[Tomnick]

To elaborate slightly on the above, 'blocking back inside home signal' (2-4) is sent to obtain permission for various stationary obstructions within the clearing point (including a train, work affecting the safety of the line, possession protection, but not a train movement that won't come to a stand within the clearing point) and 'blocking back outside' (3-3) is sent solely for a train movement going outside the home signal, and back into the rear section. The receiving box will acknowledge it by repetition (if it's been sent legitimately, without a train already in section, there's no reason why 2-4 should be refused and only one that I can think of for 3-3 to be refused, and that relates solely to short sections and has been taken out of the signalling regs because it applies at so few locations, being instead added to the box instructions at each), and the sending box will place the block indicator at 'train on line'. It's removed by 'obstruction removed' (now 2-1-2, used to be 2-1) when the line is again clear to the clearing point.

 

[Smudger105e]

Thank you both. For someone who is pretty green on these matters, these posts were useful.

 

[Tomnick]

No problem, it's always good to see folk taking an interest in the finer details of signalling .

 

[Smudger105e]

I had guessed it was to do with protecting the obstruction, but couldn't see why it would need to be done, but I hadn't sussed out that when a section has been cleared, the signalman can't just put it back to train in section or line blocked.

 

[Tomnick]

Indeed, it's effectively an extra layer of protection - not only do you have the signalman with the obstruction (who knows that he shouldn't accept a train from the box in rear), you now also have the chap in rear (who knows, but might not notice the block flick to TOL on its own, that he shouldn't offer the train in the first place).

 

[edwin_m]

If the signalman at the end of the section could just put the block to train on line without alerting the one at the beginning, then the signalman at the beginning might assume his colleague had forgotten to replace the block to Line Blocked after the previous train, and ring up to ask. At which point the signalman at the end of the section might have a brain fade and just release the block instrument.

 

[sbt]

At which point the signalman at the end of the section might have a brain fade and just release the block instrument.

It strikes me that half (or more) the contents of The Rule Book are there to mitigate the consequences of real people failing to follow the other half. Its not just in mechanical and electrical systems that the principal of 'Defence in Depth' is used.

I'm not a Signaller or anything, just someone with good Google-Fu and 'enough knowledge to be dangerous' BTW.

 

[Tomnick]

I'm not sure that such content amounts to half the Rule Book, but yes, a lot of the Rules & Regs recognise the potential for human error and introduce safeguards in mitigation! Perhaps the most obvious (and of relevance here) are those relating to the clearing point, or, more generally, the overlap - because it's recognised that it's inevitable that (for whatever reason) a train will overrun a signal at danger at some point! What you can and can't do (more of the latter!) inside the clearing point, with a train accepted, does serve as an effective illustration of the relative assessment of risk over many decades of railway operation.

 

[John Webb]

If Smudger105e likes to take a trip up the MML to St Albans and the preserved St Albans South signal box, a couple of our more complex simulations involve 'blocking-back', I recall, both inside and outside the home signals to allow certain moves to take place. (See http://www.tlr.ltd.uk/sigbox/opendays/calendar2.eb for information on our open afternoons.)

 

[Smudger105e]

If Smudger105e likes to take a trip up the MML to St Albans and the preserved St Albans South signal box, a couple of our more complex simulations involve 'blocking-back', I recall, both inside and outside the home signals to allow certain moves to take place. (See http://www.tlr.ltd.uk/sigbox/opendays/calendar2.eb for information on our open afternoons.)

I might well do that later in the year. Thank you.

 

[John Webb]

I might well do that later in the year. Thank you.

My pleasure - but avoid our extended open days in September for the Heritage Open Days weekend, we'll be too busy with the public to run anything different!

 

[ChrisPSR]

My pleasure - but avoid our extended open days in September for the Heritage Open Days weekend, we'll be too busy with the public to run anything different!

John that was a very useful description of blocking back and made absolute sense. Is it possible that the self same rulings would have applied to the Quintinshill/Caledonian boxes in 1915.

Chris Spreckley.

 

[83G/84D]

If blocking back outside home signal (3-3) in advance of an intermediate block section remind the signalman (or woman) who controls the signals in the I.B. Section not to let a train up to the Intermediate block signal.

This is because a train detained at an I.B. signal where a phone is fouund to be faulty can pass the signal on their own authority. You would not want this when you have a movemement outside the home signal!

Not so much an issue now with NRN/ CSR / GSM-R.