SPAD signals- Colour

[driver9000]

My recollection of the rulebook is that if a driver sees a SPAD indicator lit they MUST stop their train immediately (i.e. it doesn't need to refer to your line).

Correct. If a Driver sees any SPAD indicator illuminated then they must bring their train to a stand immediately.

 

[142094]

Are most SPAD indicators in locations where trains will be travelling slow (i.e. a main station)?

They are mostly after a platform starter as the train would have to stop short of the SPAD indicator - would be useless if it came on and the train was able to keep going and foul the track ahead.

 

[O L Leigh]

Not sure how they're still necessary to be honest.

*Snipped*

The only possible benefits I can see with SPAD-Is are in the marginal cases such as where a SPAD occurs after another driver has already passed over the SPAD-I AWS ramp but the indicator is still in the driver's field of vision, or a driver is on an adjacent non-signalled line but can still see a SPAD-I.

It is because I'm afraid you have not understood the purpose for which they are provided.

SPAD-Is are used were there is a point of conflict, such as a converging junction or a section of single line, where a train passing the protecting signal at danger may not be stopped before reaching the point of conflict. They are not there for the benefit of the driver of the train who has had the SPAD but for the drivers of any trains on the converging lines.

To illustrate:

Train 1 and Train 2 are both approaching a converging junction where the lines they are travelling along merge into one. Train 1 has the road and is travelling on greens but Train 2 will be held at the junction signal to let Train 1 pass first. The overlaps of the junction signals are short, so SPAD-Is are provided on each line.

Unfortunately the driver of Train 2 isn't paying attention and is simply cancelling the AWS but failing to reduce the speed of the train. He SPADs the junction signal at full linespeed and the TPWS automatically intervenes. However, because the overlap is short, his train may not stop before reaching the point where it conflicts with movements on other lines. So how do you prevent a collision between Train 1 and Train 2? Well, Train 2 will have set off BOTH SPAD-Is on BOTH lines. The driver of Train 1 will see this and immediately stop his train, thereby hopefully avoiding a collision.

This is the point of SPAD-Is. They are not there to stop a train that has SPAD-ed a signal but to stop ALL train movements heading towards the point of conflict. TPWS cannot do this, nor can AWS, because there is no way to know where to place the track equipment. Train 1 in my example could be anywhere and Train 2 SPAD-ing the junction signal will not necessarily cause the signals on Train 1's line to revert to danger. Therefore there has to be some other indicator for the driver, and it has to be a visual one.

O L Leigh

 

[MrC]

It is because I'm afraid you have not understood the purpose for which they are provided.

SPAD-Is are used were there is a point of conflict, such as a converging junction or a section of single line, where a train passing the protecting signal at danger may not be stopped before reaching the point of conflict. They are not there for the benefit of the driver of the train who has had the SPAD but for the drivers of any trains on the converging lines.

Fully understand that and have done for years thanks. However in the SPAD-I installations I've seen, all lines where a SPAD-I was visible from have been relatively low speed and had protecting signals (eg platform starters or junction signals) where the speed of any train was low enough to stop in the overlap and before a conflict point. TPWS can give greater protection here mainly because it enforces a brake application and approach speeds, and is more likely to give protection in a head-on.

If there are SPAD-I installations not covered by this then yes fair enough but TBH I'd then expect other protection methods such as double-blocking or flank protection/catch points to be used in preference to SPAD-Is.

 

[O L Leigh]

Sometimes there isn't the space for alternative protection arrangements, hence the need for a SPAD-I. This is certainly the case at the locations on our routes where SPAD-Is are installed. Where there is adequate space and the protecting signal is sufficiently far away from a point of conflict SPAD-Is are not provided.

For example, there are SPAD-Is protecting the single line section through Stansted Airport tunnel but not through Ware station. The arrangements are the same but there is a greater distance between the protecting signals and the points of conflict at Ware than at Stansted. Likewise with the triangle of junctions formed by Clapton and Coppermill Junctions where SPAD-Is are provided and the Stansted Triangle where they are not.

**EDIT**

The other thing to bear in mind (and this is where SPAD-Is really work) is that TPWS will only stop the train that SPADs the protecting signal. No train on a converging line will get a TPWS intervention because they have not passed any signal at danger. Therefore if the train that SPAD-ed the signal stops foul there would be nothing to prevent a collision.

O L Leigh