Signalling schemes-signal sighting & braking distance

[rodent279]

A question for the professional signalling engineers on RMWeb. 

When a signalling layout is being revised, and signals are repositioned, a signal sighting exercise is done, aiui. This I assume (correct me if im wrong) is to ensure that drivers have sufficient time to:-

• see an adverse signal
• respond to it
• the train has sufficent space to reduce speed to the desired level by some predetermined point in advance of the signal
• all the above taking into account "worst case" possibilities such as wetrail, some brakes being below par etc.

 

My question is, once the scheme is approved and implemented, are live trials run, under a line posession, with trains to determine whether the signal positioning is correct, and trains can to to a halt, or to whatever control speed is required, in the space available?

[jim.snowdon]

From my understanding of current signalling design, signal spacing is determined on the basis that the braking is initiated at the point of passing the signal, not the point at which it was sighted. That is a necessary worst case condition as the visibility of the signal in different weather conditions cannot be presumed. Early sighting is therefore a bonus.

Sighting, in the context of being able to identify the signal aspect against the background, is an important consideration, particularly when it is a stop aspect. There is also the issue of being able to see the signal at all when the train is stopped at the expected stopping point. This has become more of an issue with the way modern stock no longer has side windows convenient to the driver. Gone are the days when the driver could poke his or her head out of the cab side window and look at the miniature aspects (the 'pig's ears') on the side of the signal head.

The worst case elements are taken into account in the standards applicable to both signalling and rolling stock.

 

[coronach]

1 hour ago, rodent279 said:

A question for the professional signalling engineers on RMWeb. 

When a signalling layout is being revised, and signals are repositioned, a signal sighting exercise is done, aiui. This I assume (correct me if im wrong) is to ensure that drivers have sufficient time to:-

• see an adverse signal
• respond to it
• the train has sufficent space to reduce speed to the desired level by some predetermined point in advance of the signal
• all the above taking into account "worst case" possibilities such as wetrail, some brakes being below par etc.

 

My question is, once the scheme is approved and implemented, are live trials run, under a line posession, with trains to determine whether the signal positioning is correct, and trains can to to a halt, or to whatever control speed is required, in the space available?

The designed position of the signal is shown on the scheme plan, which takes account of the braking distance determined using a set of data based on the permissible speed and gradient. 
 

The signal sighting assessment is used to confirm that the signal and any associated indicators can be read in the operational context taking account of potential obscurations, sun light, cab sight lines, reading through risk etc. These days, the assessments are often done in an office environment using simulator technology to reduce the need for track access. 

 

For significant changes, where new signals are provided or existing signals are moved, a cab ride is performed to confirm the signals are fit for purpose

[The Stationmaster]

The system has changed massively over the years so ut really depends about theh date you are talking about.  What happens today for example is very different even from some of what happened in the 1990s.

 

SPAD (Signal Passed At Danger) Risk Assessment procedures didn't really gain much use until the very late 90s/early years of this century and are an additional part of the signal siting process which follows the actual siting decision and might require it ti be reviewed although TPWS has resulted in changes to the risk assessment process.

 

Sighting meetings have also changed a lot over the years both in terms of representation present, any equipemnt used, and teh actuall rules (such as nowadays a minimum sighting time for the Driver of an approaching train.  And as Jim Snowdon noted above there have been changes in the way braking distanc e is considered.  I would think to go through it all in detail would need refeence to quite a few deoartmental standards over many years and would probably fill a small book to get in all the variations of detail let alone changes of principles.