Signal Spacing and headway calculation

[onestrangeday]

Hi PJW:

thanks for your reply and explanation. I understand what you have wrote.

In case you are still confused by this subject, I shall try to answer more simply:

1. Calculate the signal spacing using the maximum permissible speed [see note # below] to determine the braking distance needed.

2. Calculate the headway using the timetabled speed (where that is lower).

Note #.
The train type may perhaps be physically able to travel faster than the "linespeed", but since this is not permitted at the relevant site then the maximum speed of the train itself is irrelevant.

However if a train is not capable and/or permitted to travel as fast as the line would allow other traffic to operate, then when calculating braking distance for that particular type of rolling stock, you then use the lower maximum speed of the rolling stock (the nominal "linespeed" in this case is irrelevant for this train type).

This permits a line to be signalled for 125mph trains with good braking but also utilised by trains having worse braking but that are limited to 100mph. This is OK provided that they can stop from 100mph in the same distance that a 125mph train takes to stop from that higher speed.

The choice of signalling system for the line must cater with the constraints from all the traffic types; it must deliver the stopping headway for slow trains, the non-stopping headway for the express trains and be safe for all types of traffic for the maximum speed at which they are permitted (taking into account both the line and the rolling stock constraints).

Hope this clarifies.

Hi Signalling Professional:

I have questions regarding the calculation for signal spacing and headway and I would like to correct myself if I am wrong.

Situation:
Line speed is 100km/h, max train speed is 120km/h, required headway 3min

My question is which speed of the above do I use for calculating the signal spacing and headway.

My understanding is that:
a) when calculating the signal spacing use line speed 100km/h (that's the speed allowed to run the train normally)

b) when calculating what signalling system to be used for the railway system to suit the required headway uses the train speed (120km/h) that's the speed the train could possibly run on the line.

Maybe this is simple question, but am I right for this ?



thanks