Question about Absolute Block Signalling

[158801]

Imagine a fairly mundane two track railway controlled by Absolute Block signalling. No junctions or sidings. Just plain track.

Along the route are manual signal boxes - let’s say spaced every 10 miles. Trains run once every two hours.

Train one leaves the section controlled by signal box “A”, into the area operated by “B” then onto “C” and so on.

Train two is now operating. And I’m curious about the order of events. Train two now needs to leave the area controlled box box “B” heading to box “C”.

Does box “B” have to request line is clear from box “C” immediately before sending train two on its way or does box “C” have to give line clear as soon as train one has passed.

I suppose in a nutshell I’d like to know who is supposed to take the initiative - box “b” for requesting the line is clear or box “C” for saying the line is clear ?

 

[7Paul7]

When box b lets box c know train 1 has entered the section between the 2 boxes box c will move the block instrument to train on line. This prevents box b requesting a line clear until train 1 has left the section complete with tail lamp and they have received out of section from box c and the block instrument is back in the normal position

Once the block is in normal box b would then request a new line clear to be able to send train 2 into the section

 

[John Webb]

An important concept, introduced mainly as a result of the Abbot's Ripton disaster in 1876 where signals had been left at 'clear' and froze in that position, was to return signals to 'danger' as soon as passed by a train, and keep them at danger. Only when permission had been given for the next train to proceed would the signals be cleared to allow the train to pass. Thus if the signals became frozen in position it was more likely to be in a 'fail-safe' position.