2004 Part B Question 5

[PJW]

Hello Peter,

Please go through my answer for the aspect sequence question for 2004.

The one issue in this question is the transition for two 3 aspect signals to 4 aspect which in my view do not need approach release since trains are starting from a stand. Hence I have put down my rationale for doing so on the chart. Please let me know if this is correct as this matter is not mentioned/addressed in Appendix S which you posted on this website.

Thank you again for your time and attention

Regards

Somehow I missed this one; now I've found it I'll look at and respond.

In brief however there is no need (or indeed value) in approach releasing. However at any transition from a 3 aspect line (including terminal platform) if the train gets a Green and then joins a 4 aspect line, it is important to satisfy yourself that if the driver gets a Yellow at the next signal that there is sufficient braking to the Red. It depends on the attainable speed which the train can have reached (worse caase is short train as it can acccelerate sooner once clear of junction) and the actual signal positioning. In practice it is usually OK given that there is approx 0.5 SBD at max permissible speed and braking distance is proportional to the speed squared. As a crude example 70x70=4900 and 50x50=2500, so very roughly the distance that is half braking distance for 70mph would be nearly enough braking distance at 50mph; if the turnout giving access to a 70mph line were only 25mph therewould be little need to worry given the length needed for a train to accelerate, but if the turnout were 40mph then calculation certainly required to see if OK. If not there would need to be earlier warning; in general this would be achived by making what would have been the 3 aspect signal protectng the junction, 4 aspect instead. Approach release which you are thinking about is "modified sequence" and generally has to be used where there is a transition that is occurig on plain line rather than at a junction between different lines.

I will go back and look at Appendix S to clarify when I get a chance.