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TWO REDS RULE
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31-08-2010, 10:14 AM
Hi PJW,
Can you please tell me about TWO REDS RULE? On which condition and how we will apply this rule? Thanks & regards Varalakshmi Penneru  (31-08-2010, 10:14 AM)Varalakshmi P Wrote: Hi PJW, I am afraid this is quite complicated and you may regret having asked. It really needs some diagrams that I have not time just now to draw, but here goes a text explanation. Double Reds Relatively (>last 10 years) new thing to NR. Basically in the past NR never had any train protection, then after some accidents fitted TPWS. Although not a 100% safety system, for the first time TPWS provision gives some form of certainty of how far a train could go if it SPADed. At the retrofit programme, TPWS consisted of: a) a TSS at the signal and, b) where necessary, an OSS as a speed trap on the approach to the signal that would intervene if it was clear that the train going too fast to be able to stop at the signal (i.e. it acts upon anticipation that a SPAD will be inevitable). Broadly (depending upon the type of rolling stock vehicles and the actual length of overlap gradient etc that exists) then this concept basically works ok for a passenger train running at up to about 60-70mph. TPWS was conceived as a
PJW
Hi PJW,
Thanks for the information. I have the diagram and now I am clear about TWO REDS. Thanks & regards Varalakshmi Penneru (31-08-2010, 01:13 PM)PJW Wrote: [quote='Varalakshmi P' pid='1989' dateline='1283246087'] I am afraid this is quite complicated and you may regret having asked. It really needs some diagrams that I have not time just now to draw, but here goes a text explanation............................................. Basically the short version is FORGET IT for the vast majority of situations; only applicable if line speed quite high. Certainly not a high priority to understand for the IRSE Exam Control Tables- in fact if you declare your practices are prior to the year 2000, then it won
01-09-2010, 09:45 AM
(01-09-2010, 05:39 AM)Varalakshmi P Wrote: Hi PJW, I am afraid this is quite complicated and you may regret having asked. It really needs some diagrams that I have not time just now to draw, but here goes a text explanation............................................. Basically the short version is FORGET IT for the vast majority of situations; only applicable if line speed quite high. Certainly not a high priority to understand for the IRSE Exam Control Tables- in fact if you declare your practices are prior to the year 2000, then it won
PJW
26-11-2010, 06:03 PM
A little bit more to add about the two reds rule.
The concept in TPWS fitment is that for any OSS placed on the approach to a signal, the position/ speed setting of the OSS is chosen to lie on an imaginary deceleration curve, approximating to 7.5%g, leading to a stand at the post of the signal. The premise for this is that the "defensive driving" policy requires a driver to make a (final) approach to a red signal using no more harsh a brake rate than 6%g, even if the driver knows that the train brakes have a better performance than this. This means that any train which finds itself on or faster than the 7.5%g line on the approach to a red signal is probably in danger of passing that signal at danger, and certainly violating the defensive driving policy. The intention is that TPWS will not intervene when a driver is acting correctly. The fact that a TPWS intervention has occurred is recorded on board the train, so the driver in question has to explain this fact to their line manager. Therefore, any TPWS intervention can "undoubtedly" be said to be due to a driver violation, whether this be a truly unsafe approach to a red signal, or only a breach of defensive driving policy on a train with good brakes! Where a signal has higher speed approaches, it becomes necessary to provide multiple OSSs on the approach to the signal, such that 1. A train attempting to approach the signal at full permitted speed without any attempt to brake will be "safely managed" to come to a stand, at emergency brake rate (typically 12%g) , at or before the point of conflict protected by the signal. 2. The residual speed after a train has just avoided tripping at each OSS is able to be safely managed in a similar way by the next OSS or TSS device. There should in theory be no limit upon the number of addtional OSS, OSS+, OSS++ etc applied on the approach to the signal in question. If these additional OSS systems happen to fall on the approach to another signal ("TPWS outer signal"), then there is in principle no problem, since they will never attempt to enforce a braking curve more restrictive than that required by the outer signal's aspect anyway. Remembering that TPWS is typically only fitted at "high risk" signals protecting convergences or head-on conflicts, the "outer signal" may well not qualify for TPWS fitment at all. However, there is still no fundamental problem in OSS+(++...) devices for one signal happening to lie on the approach to another signal further on the approach. You'll see that I have strongly argued above that the "two reds rule" should not exist, but it clearly does, and for very good reason... The issue lies in the fact that in practice, different trains have widely varying braking characteristics: 1. Some higher speed passenger trains have a "9%g mean" service brake rate - the driver will need to apply a harsher brake rate on the outer approaches to a signal, in order to comply with "defensive driving" on the final approaches. 2. the passenger / freight OSS timer adjustment is just a crude 2-state adjustment which does not account for this. 3. The further you go back on the approach to the stop signal, the more difficult it is for the driver to judge what is "defensive driving" anyway The result of these issues is that, the further you place an OSS back from its signal, the greater the probability of getting "false trips", because the distinction between "safe" and "unsafe" behaviour has blurred. The 2 reds rule solves this problem by artificially holding the "outer signal" at red when the "inner signal" is protecting a TPWS-mitigatable conflict. The outer signal's OSS is positioned: 1. On the final approach zone of the outer signal's 7.5%g approach curve. 2. To to be able to stop a full-speed train, at emergency brake rate , by the point of conflict of the "inner signal". 1. removes the tripping uncertainly associated with an OSS which is a long way distant from it's stop signal 2. ensures that a "worst case" violation can still be trapped. So the two reds rule is a fiddle to manage a conflict between operating rules and train protection requirements on a mixed-traffic railway. Reuben |
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