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+----- Thread: 2014 Q9 Junction Signal on main line (/showthread.php?tid=2229)
2014 Q9 Junction Signal on main line - PJW - 04-10-2017
Another I have been sent recently; not sure if I am going to find time to review in remaining time so posting here in case anyone else gets the chance.
However by chance I have responded by email to someone else who asked for help on part c of the question, so I am copying the email that I sent in response:
Controls:
- Approach Release from Red MAR.
Expectation of clearance on approach. SPAD trap. The later we leave the release (may have to because risk of driver seeing the proceed main aspect without the associated route indicator that qualifies it to mean a lower permissible speed) the worse it is.
- Approach Release from Yellow with flashing aspect MAY-FA.
Colwich accident re signal ahead SPAD trap.
Confusion when flashing sequences overlap.
- Approach Release from Yellow MAY-YY.
A dodgy form of signalling reliant on a bit of a white lie ;really some attempted “speed signalling” as a bodge. OK we “get away with it” when customising a re-signalling for the attainable speed / braking profile of particular rolling stock; then someone introduces a train with BETTER braking that the drivers learn to utilise and that INCREASEs the risk as they encounter junction set for a diverging route too fast, whereas they were actually braking sensibly to stop at the signal that the aspect sequence had told them they were due to stop at.
- Main Aspect Free. MAF
Didcot Foxall accident- nowadays signals with one route at end of reversible is completely (or at least partially) signalled as if junction.
- Main Aspect Free / MAF-Splitting Distants.
Also for other situations at high speed diverging junction, driver may not know which way they are going until too late; could be more than inconvenience of an electric train disappearing onto an electrified route or driver sent on a route for which have no route knowledge, it could be a diesel HST routed into Heathrow tunnel which only designed for electric trains so no air extraction / fire suppressant, or even worse a train signalled onto a line for which it is foul of structure gauge such as shuttle train leaving Eurotunnel and encountering the first bridge on the NR classic network.
Talk about utilisation of splitting banner, but in particular PRI
Good example might be District line (surface stock) being routed onto the Piccadilly line that initially is just a 4-track overground section until Earl’s Court and then disappears underground into a tube tunnel of less diameter than the District line train. OK certainly not the sort of railway one would expect a splitting distant(!) but it gives a dramatic illustration of the idea. For this, the original mitigation was that before the tunnel an out of gauge train would have broken a glass tube which contained mercury and was in use to complete an electrical circuit; this was then used to disconnect traction power. Nowadays technology allows height to be checked in a manner that does not involve contaminating the environment with a hazardous substance.
Could also talk about through-reading controls, one signal requiring the next signal off and the consequential need to extend the A/L lookback further from the junction signal
Special case could be "Conditional Double Red" which makes the junction safer by using the TPWS OSS of the outer protecting signal, but that does confuse signallers regarding the extent of aspect reversion if they should cancel a route and therefore the secondary risks from driver’s reaction, emergency braking, disruption after what was a “safe SPAD” etc.
Worth bearing in mind and working into your answer somehow is the trueism that unless a signal is at Red then a SPAD is impossible, so therefore more a signal is at red the greater the chances must be that one of the “train approaches a red signal” scenario will actually result in a SPAD; therefore whereas we think that putting a signal to red must reduce risk, this is not always true….
Above is not a direct answer to the question portion, but should have plenty of material to enable you to answer it fully.
RE: 2014 Q9 Junction Signal on main line - sowmya.ael@gmail.com - 05-10-2017
Please find the review of the attempted answer by PJW which was to sent me through Email.
Even though he couldn't give very detailed comments due to time constraint, this will surely be useful to gather some additional points/correct the mistakes made.
Thank You PJW for your time and efforts put in to guide us.