Reduced overlap

[PJW]

Hi PJW,

Could you please explain the meaning of Reduced overlap supported with circuits?

The modern use of the term reduced overlap is merely an overlap shorter than a full overlap (generally 180m but on certain higher speed lines 225m) that can be justified as suitable in the circumstances. This is generally a function of permissible speed but also takes account of the site conditions and the effectivelness of train protection. There are no special circuits; it is just that the overlap track circuit is not as long as otherwise it might be and points lying outside the reduced overlap are not locked. There would not be a full overlap for that signal.

I suspect you meant what is nowadays called a restricted overlap. This is ADDITIONAL generally to a full overlap and is selected for use when the full overlap would be restrictive to other traffic. The signal in rear authorising moves up to the signal having the ROL, is provided with a Warning Route; this just proves a sub-set of the full overlap controls but imposes quite stringemt approache release of the aspect to ensure that the train passes it slowly. The only special circuits affect the subsequent "stepping up" of the Warning route to the Main route in circumstances in which the forward route from the exit signal is latter set and thus commits the full overlap for the signal in rear.
Basically the (W)RLR has to be dropped the (W)NLR picked then the (M)RLR picked. The important thing is that the aspect isn't replaced during this; the reason why
a) have to prove that the full overlap ASPECT conditions are satisfied
b) In RRI only: the signal hasn't actually yet cleared (in SSI the change of the class of route can be exerchanged in a single instruction before the message to the TFM is generated and thus "invisible").