2015 Module 3 Question 8

[mattslade222]

Hi,

I have been talking this question through with my sponsor and on reflection feel like I may have made a complete hash of it! My attempt is attached below.

I'm not looking for a full critique but a pointer in relation to parts A and B, whether I have interpreted the question correctly.

I read A as why can a train protection not always stop a train at a restrictive signal. So in the case of TPWS it doesn't know a SPAD is going to happen until it actually happens, OSS improves on this but its not fool proof. Continuous ATP goes along way to improve, however, it still cannot account for some aspects. Or even something like the signal being reverted at the last minute.

Questions B i read as, once a trigger of the system has occurred why may the train not be stopped prior to the overlap at fouling point.

Thanks for any responses.

[dorothy.pipet]

I've not really read your answer but you need to think as widely as possible so some disjointed thoughts for part A:
- Track equipment might be incorrectly installed - ATP/ETCS balises missing or mis-programmed
- some other fault on trackside such as loss of power (TPWS)
- on-train equipment may malfunction or be isolated/overide/wrong mode - this could be for a genuine reason such as calling past a signal
- or the breaking performance is out of spec - adhesion such as leaves/ice/oil on the line, poor maintenance and so on.
- operational stuff i.e. overspeed which TPWS does not catch on the approach,
- or signal replaced in front of a train,
- or driver error doesn't see /respond to caution aspects
- or driver cancels TPWS warning without braking
- poor design e.g. insufficient OSS loops or gradient not taken into account, or insufficient balises (odometry not reset frequently enough so within cumulative tolerances).

Then section B it depends how you define the system, but I think it implies where the equipment is working but the conditions/human are not, so
- adhesion, override,
- TPWS worst case is when the driver passes an OSS just under the trigger speed then accelerates.
- if the signal is replaced in front of the train for ETCS the route locking is held so the train will pass the hazard point but it will be safe to do so.
I would interpret that the 2nd part wants a little more detail on the specific items. I see no reason not to include the cases like ETCS where the SPADding train is protected

If you want to talk about overlaps then you need to explain how that "translates" to end of movement authority and hazardous location in the question - consider: the overlap on an Automatic signal does not mean there is a hazard after it.

[Jerry1237]

Matt,

Am struggling to read your attempt so am going to throw some thoughts down.

Assuming a stop signal is the end of the authority and we take tripcocks, AWS and TPWS OSS as examples, none guarantee stopping at the signal. Both should stop a train within the "overlap" under normal braking conditions. All could end up as a SPAD. Think about AWS, at linespeed, let's assume 100mph, would 180m be sufficient to stop at the signal or even in the overlap? Is ATP a solution? Whilst it may improve various circumstances, equipment failing in front of the train will not necessarily prevent a SPAD or movement beyond a MA - remember, not all ATP is ETCS!

Does that mean there will be a collision? No. Remember the difference between "hazardous location or conflict" and what a SPAD or exceeding a MA actually are.

For part B apart from equipment failure, there is adhesion, driver error (accelerating after an OSS [see Dorothy's comment]), notice some of these are mentioned in your answer but i) mixes up overlap and hazard/conflict.

[mattslade222]

Thanks to both of you for reviewing and providing comments, I appreciate you taking the time. Apologies Jerry a) for the shoddy handwriting, b) the poor scan from my phone. I promise its legible in real life!

My issue is that depending on they system:

• TPWS - TSS/OSS
  
• Tripcocks
  
• ETCS level 1 - with infill
  
• ETCS level 1 - balise only
  
• Modern CBTC with continuous ATP
  
• etc
  

The answers for a) and b) can overlap significantly. For the continuous systems where they are design to stop you at a signal, the reasons for a and b are almost identical. This is making me unclear on exactly what the examiners are after from each part.

Anyway, I have taken both your comments on-board and produced a similarly shoddily scanned attempt - no need to review if you don't wish. (For some reason the website wont let me upload the attempt, Ill try again later!)

You both mentioned the difference between overlap and hazardous location. I may be getting this wrong, but the hazardous location and the end of the overlap are the same thing. If you are at a controlled signal, past the overlap you could come into conflict with a train on another route. For both controlled and auto signals, a train may have stopped just beyond the overlap (although unlikely). Am interpreting the hazardous location incorrectly or should I have just explained it?

Thanks very much to you both.

[dorothy.pipet]

My understanding differs from yours:
If the overlap is at the clearance point of points then yes it is the hazardous location, but ...
Perhaps this signal is a controlled signal (route) because of points in the overlap of the exit signal - in this case the hazardous location is not at this signal's overlap but after the exit signal.
Perhaps it's a controlled signal because there is a Level Crossing in the route, in which case that hazard (the level crossing) could be anywhere (almost) between that overlap joint and the next signal
Perhaps there are no points but it's bi-directional working; in some cases there might be a bang-road limit of shunt in route somewhere, the conflict point would be the overlap of that LOS.

[mattslade222]

My understanding differs from yours:
If the overlap is at the clearance point of points then yes it is the hazardous location, but ...
Perhaps this signal is a controlled signal (route) because of points in the overlap of the exit signal - in this case the hazardous location is not at this signal's overlap but after the exit signal.
Perhaps it's a controlled signal because there is a Level Crossing in the route, in which case that hazard (the level crossing) could be anywhere (almost) between that overlap joint and the next signal
Perhaps there are no points but it's bi-directional working; in some cases there might be a bang-road limit of shunt in route somewhere, the conflict point would be the overlap of that LOS.

I see, thank you. I'm not thinking wide enough!

Thanks very much.

[Jerry1237]

From your response Matt, you need to define and discuss one system because, as you say, they differ and contrast to each other. However, reminder to "read the question"; "A train protection... "