How does bi-di signalling work?

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najaB

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Another basic signalling question - how does bi-di work in terms of signal aspects? I'm heading south from Edinburgh and noticed that on the approach to Grantshouse the bi-di signalling had a conventional G-YY-Y-R sequence.

So how does this tie in with the right-road signals to avoid a green in both directions at the same time? Is it (pardon the crude ASCII art):

G->YY->Y->R-R<-Y<-YY<-G (i.e. red at either end of a single block section) or:
G->YY->Y->R-R-R<-Y-<-YY<-G (i.e. an 'empty' section between the two reds)?
 
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Railsigns

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The Grantshouse-Innerwick bi-di signalling [*] is non-standard in that the automatic signals for both directions can show proceed aspects at the same time. As a train approaches in one direction, you can watch the signals for the opposite direction step down G->YY->Y->R. In a standard bi-di installation, only the signals for one direction can clear, the others being held at red. The interlocking only allows the direction of signalled running to be reversed when the entire bi-di section is unoccupied.

[*] - On the Down line only; the Up line has SIMBIDS - the only SIMBIDS in Scotland.
 

Tomnick

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The integrity of the reversible line, or indeed a single line worked in the same manner, is achieved by holding the 'protecting' signal (so usually the one in rear of the crossover(s) that give access to it at danger once a train is signalled from the opposite end until the whole length of the reversible line is clear, with 'direction of flow' indicators to show the current state.

What happens to any intermediate signals is less important - sometimes they'll be placed to (or maintained at) danger for the whole length of reversible line as soon as a route is set in the opposite direction, and sometimes they'll work as true auto signals and stay cleared until a train actually approaches in the opposite direction (then each will step down from G to YY, to Y and to R as that train occupies track circuits in turn and finally back to G as it clears the track circuit in advance of each signal. It doesn't really matter, as the interlocking should prevent any train ever approaching any of these intermediate signals whilst this is happening.
 

edwin_m

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With relay-based signalling the logic to work out the aspects of signals is done in trackside cabinets. For most automatic signals the only communication with the signalling centre is the state of occupation of the track circuit (although some also have a replacement switch on the panel which may also confirm back that the signal is lit and the replacement has operated). Thus restoring these to red when a route is set in the other direction would require at least one extra cable core connecting all the signals, and cable cores cost money.

With solid-state and computer-based interlockings all the logic is done within the central interlocking so holding them at danger for moves in the other direction is simply a matter of geographic data with no extra hardware needed.
 
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