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4 aspect signal spacing
I have been asked the following:

what is the maximum signal spacing between two signals in 4-aspect signalling?

The simple answer would be half of the maximum over braking that your railway authority allows. Eg if you are allowed one third over braking, the max between evenly spaced 4 aspect signals, would be two thirds of BD. The illustration below assumes 1/3 over braking is max allowed.

However, there is some leeway for inconsistency of spacing on 4 aspect and the normal is that the max variance between successive four aspect signals is one third two thirds as opposed to half and half. Under this scenario, you could have up to eight ninths of of braking distance followed by four ninths.


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