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Headway assumption / calculation and choice of signalling system
Hi All,

I have couple of questions in terms of the IRSE exam for Module 2 and would appreciate if somebody could provide clarification. I read the Study pack and the forum as well, but haven't found the answers of these yet.

Based on the previous exams, the headway can be specified in two ways. Explicitly stated for different train types (stopping, non-stopping) in train per hour, or listing all possible movements between stations (and/or junctions) for different train types again (freight, non-stop passenger, stop passenger on mainline, branch, etc.) in train per hours or days. Sometimes both are given, but sometimes only the latter. If only the latter (e.g. mod2 exam 2017 layout 1), is there a way to calculate the overall required minimum headway and what is this depend on? What I mean is that I would e.g. pick the most busy junction, the "bottleneck" and based on the given runs count the trains running, turning, etc. through or at this junction in an hour, and state this as an assumption.

Choice of signalling system:
I understand that in most of the cases, to satisfy both the minimum signal spacing (max braking distance), the headway and operational requirements, both 3 and 4 aspect signalling (or isolated 3 aspect and continuous 3 aspect) will be required. However, the signalling principle I working with (South Australia) supports only 3 aspect signalling as far as I know. As the distance between signals in 3 aspect signalling is minimum braking distance, this is sometimes (most of the earlier exam layout, e.g. mod 2 exam 2017 layout 1 - station C) not possible on the given speed and traffic at complex junctions. The question is if the statement of applied principle and country is enough to cover the above together with slowing down the trains (or certain type of trains - non-stop) to an acceptable speed (either with specific aspect sequencing or speed boards, or any other way) or is it and expectation from the examiners that minimum two signalling solution is used?

Any clarification would be much appreciated.

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Headway assumption / calculation and choice of signalling system - by gnemeth1980 - 17-08-2019, 07:22 AM

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