3 aspect distant signal

[Lee Rambo]

This sounds like an exam question - please show your workings.

 

[Efini92]

Examples:

- Where one signal in an otherwise typical run of four aspect signals isn’t capable of displaying a red aspect (perhaps it’s in a tunnel).

- In the transition from three-aspect to four-aspect signalling, where the last three-aspect stop signal is R/YY/G and followed by a three-aspect (Y/YY/G) to provide the correct aspect sequence and braking distances for the first couple of four-aspect stop signals.

- Approaching a four-aspect stop signal from an AB section or a long TCB section (either a single three-aspect distant signal or an outer and an inner distant).

- Where it’s necessary to provide an outer and an inner distant signal approaching a two- or three-aspect stop signal (e.g. the outer, YY/G, is needed to provide the braking distance, but a station intervenes and its desirable to have an inner distant to allow the level crossing beyond the stop signal to be kept open to road traffic for longer).

...and probably more besides!

Having a green 3 aspect then a single yellow at the first 4 is a correct sequence

 

[Tomnick]

Having a green 3 aspect then a single yellow at the first 4 is a correct sequence

It is (and I know a few places where it happens), but the problem is that if the second four aspect signal is at danger, you have excessive distance from the YY to the R, which is a risk in itself. Inserting a three-aspect distant before the first four-aspect stop signal solves that one (and seems to be the preferred approach in more recent schemes).

 

[Efini92]

It is (and I know a few places where it happens), but the problem is that if the second four aspect signal is at danger, you have excessive distance from the YY to the R, which is a risk in itself. Inserting a three-aspect distant before the first four-aspect stop signal solves that one (and seems to be the preferred approach in more recent schemes).

It’s the second 4 aspect where they put the distant. All it does is increase the distance between two trains.

 

[Tomnick]

It’s the second 4 aspect where they put the distant. All it does is increase the distance between two trains.

Yes, sorry, the distant would come after the first four aspect stop signal (which then becomes three aspect, R/YY/G). It means there’s potential for slightly reduced headway as that signal would then be able to show a green sooner, although it’ll still be constrained by the three-aspect section in rear.

A quick sketch...

 

[carriageline]

They are also used if there is not sufficient braking distance between the single yellow and the red in 3 aspect areas. So by giving the 3 aspect distant, you advise the driver of the single yellow earlier and keep block sections short

 

[MarkyT]

They are also used if there is not sufficient braking distance between the single yellow and the red in 3 aspect areas. So by giving the 3 aspect distant, you advise the driver of the single yellow earlier and keep block sections short

Very common in cases where a simple former semaphore layout was converted to colour lights while retaining legacy signal positions and lever frame control.

 

[gimmea50anyday]

Between micklefield and Church Fenton there are several 3 and 2 aspect distants. The are is MAS, however there are only 2 signalling sections in each direction and are effectively 3 signals in length. There are also 4 aspect signals that will never display anything other than red and green because the following signal is a 3 aspect distant. Think it’s historical as to the reason or bizarre BR money saving but does cause capacity constraints.