Fouling point

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wilric

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Is there a set distance that a fouling point has to be after a signal? eg if a train passes a red signal does the train have to have enough room to brake and stop before the chance of crossing into the path of another train?
 
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GB

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Usually there is an overlap of 185m in TCB areas beyond the signal. If the point of conflict is within this distance and a route set through it, the train for which the overlap is applies will either get held at the previous red signal (ie the signal before the signal that protects the conflict), or there might be a "warner route" available. This means the train will approach a red signal, will clear to single yellow and the train will stop at the signal that is protecting the conflict. This is designed to bring the train down to a suitable speed.
 

wilric

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What if there has been no route set through. Would the train still receive a red at the previous signal to the signal protecting the conflict?
 

GB

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If there was no route set through the over lap you could bring the train to a stand at the protecting signal, however you won't be able to set a conflicting route through the overlap untill the train has come to a stand at the protecting signal and the overlap timed out.

Hope that makes a bit of sense!:)
 

wilric

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Thanks it makes sense.
What I am getting at is.. whether an automatic train protection system is foolproof eg if a driver goes through a red light where no route has been set after the signal and there is then not enough braking distance before the fouling point there is still a potential for a collision, correct?
 

GB

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I'd say no system is foolproof and I guess it will depend on circumstances. But most sites where it is calculated that a collision may occur in the event of a spad are usually fitted with TPWS which is designed to stop the train within the overlap.

If no route has been set after the signal or through the overlap, the overlap will be clear anyway.

Edit,

So with this picture as a basic example...

With a route set from 3 to 4 you won't be able to set a route from 2 to 1. Train will be held a 2. Similarly, if a route is set from 1 to 4, you can't set a route from 2 to 3, train will be held at 2 again.

If no route set from 3 to 4 or 2 to 1, there will be no danger of collision as the train will be held at 2 (or the one above it). The only was to get a collision on the below pic is for a train to spad both 2 and 1...extremely unlikely.
 

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Joseph_Locke

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Just to add, if it is considered that even if no conflict exists in the signal's overlap a train passing the signal at stop could overrun the overlap into a confilct, then the preceding signal can be approach released from red to further control the train's speed of approach to the first signal (this is known as "double reds").
 

wilric

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So do places exist where if a Driver mistakenly passes the last signal at red and at the maximum allowed track speed is there then not enough room for the train protection system to stop the train before reaching the fouling point?
 

Joseph_Locke

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So do places exist where if a Driver mistakenly passes the last signal at red and at the maximum allowed track speed is there then not enough room for the train protection system to stop the train before reaching the fouling point?


Yes and no. Every signal on the railway was assessed when TPWS was introduced, and generally any signal protecting a a conflicting movement was fitted (so not ones just protecting another signal) as were many high-risk reductions in permissible speed. However, this round of fitment only gauranteed protection where the approach speed was 75mph or less (e.g. the driver was trying to stop but had got it wrong).

TPWS+ raised the protected speed to something like 90 (my memory is incomplete on this one) and TPWS mini was introduced to manage low-speed but high risk approaches to buffer-stops.

New schemes obviously include TPWS as an integral part of the layout design, but the wide range of system reaction times and braking rates can lead to some deep thinking; for instance:

Let's assume a 100mph railway carrying a mix of traffic ranging from state-of-the-art 125mph EMUs all the way down to 75mph loco-hauled freightliner trains. One one particular line the signals are four aspect and are named A, B, C and D. Signal D protects a major conflict (say a trailing and facing connection in rapid succession).

With D at Stop, C will be single yellow, B at double yellow and A at green. A probably wouldn't be fitted with TPWS at all as it wouldn't do any good to test the train speed here.

B (double yellow) similarly wouldn't have TPWS as this signal is the first point a driver should brake, so the train speed wouldn't be reducing anyway.

C would have an TPWS+ overspeed sensor (OSS) to test that the train speed is actually reducing, but because it is roughly half way through the braking run it is likely to be set to something like 75mph (if the train is going faster than this the brakes come on automatically)

D would have an OSS on the approach (set to something like 25mph) and train stop sensor (TSS) as a last resort, leaving 200m of overlap to stop the train in.

This is all well and good for our 12%g EMU, but consider our 75mph freight.

Our inattentive class 60 driver passes A at green (no issue), B at double yellow (again, no test applied but he should now brake) and C at single yellow. Unfortunately, the OSS here is set to 75mph, the speed he is doing anway, so it doesn't trigger an application. The first check is the OSS at A (25mph) which triggers an appication. After a princely 30 second delay the 6%g brakes apply and the freight train sails over the TSS at well over 25mph, straight into the conflict (and possibly all the way through the junction).

The only way to cure this is move all the controls back one signal. We now hold signal C at Red, so that signal B (now Y) gets an OSS. we can use timers to test how fast the train is travelling from B to C, releasing C to single yellow if all is well.

This kind of thing is the yes and no part, because there probably are places where the risks are higher and no because they're as well managed as possible.
 

Joseph_Locke

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What do you mean by "30 second delay the 6%g brakes apply"?

I understood from a colleague (who drove freight trains for a well known FOC) that there is a delay between demanding a brake application and actually getting one on a long freight, and the 6%g was from memory (0.58m/s/s) for typical freight braking performance.

If I'm wrong, correct me and be glad I'm not a signalling engineer!
 

GB

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Ah right, yes I see. On a long freight with the brake distributor timings set in "goods" the brakes are slightly slower to apply and release and with it being a long train, the brakes will apply towards the front quicker than the back. In an emergency situation your only talking a few seconds delay for full emergency braking efficiency though.

You'd be surprised at actually how quick they can stop when they need to given the right conditions.
 

wilric

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What about in bi-directional multli line situations such as just before a main line station. Sometimes the movement to another line is quick rather than gradual and so hence less time for emergency braking, although the speed of the train will be less. Is this a more likely scenario to come under the 'yes' it is possible to pass the fouling point?
 

GB

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The same signalling principles and overlap requirements apply on bi directional lines as they do on non bi directional lines, so it should all be the same.

That said, where human nature is concerned there is always the risk of error (even if the chances are tiny).
 
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