(17-01-2011, 03:35 PM)fil Wrote: Hi all,
I can't seem to get my head around the above aspect.
the definition i have is:
STOP, applies to the movements in the direction to which the signal can be cleared; other movements may pass.
im confused because only movements in the direction to which it can be cleared will see the aspect. the other movements will see the back of the signal.
thats my thinking but obviously i am missing something.
It has to be said that there are few of these GPLs still around......
Basically they are at the exit of sidings protectiing a facing crossover to the running line.
# The "direction to which they apply" is when the crossover is reverse (and a train from the siding is authorised to proceed onto the running line).
# The "other direction" is when the crossover is normal and in this lie the driver is permitted to pass the signal displaying ON (i.e. yellow and white, or in the case of LED signals, two horizontally displayed yellows). This means that they can shunt forward into the spur or headshunt which generally runs approximately parallel to the adjaent running line before terminating in bufferstops (may be as litle as 50m but could be 200m or more).
This allows shunting to take place and get the rear of the train clear of a set of handpoints so that the train, or part of it, can be set back into a different siding. the signaller is not involved- the driver can shunt forwards and backwards as many times as desired whilst the points remain normal. You can think of a yellow shunt as being able to be ignored for shunting movements contained within the siding complex.
So you got confused because "direction" in this case meant the
lie of the points and thus the final destination of the movement passing the signal, rather than whether thether the train was facing the signal or going the opposite way!
Note that if such a GPL were provided as a normal GPL then
1. the signaller would need to route the train into the headshunt and indeed
2. there would need to be an opposite facing signal to authorise the movement back from the headshunt into the siding again.
It would also mean that the train driver would have to go the full distance in order to get behind that signal in order to be routed back. Note a driver may only actually need to go say 20m beyond the signal for the purpose of getting the back of their train clear of the points that need to be set the other way; a yellow shunt permits reversal here. However if a normal GPL were provided, then the train would have to go the full distance all the way to the opposite direction signal (which could be 200m+ away) before being permtted to change direction to come all the way back again!
Note that where a pair of opposite direction GPLs are provided like this, it is usual to permit the signaller to set them both one after the other ("opposing locking ommitted") and the train can then shuttle up and down without further intervention from the signaller, until one or other or both routes are cancelled.
