Headway means the same thing inn essence; how soon a secnd train can follow the first. Railways using ATC tend to be those for which the important thing is "stopping headway" or even mor important "platform re-occupation times". In detail it is very much a question of modelling via computer simulation.
You don't say but I am guessing that you are assuming a "moving block" system where
a) the track is not divided up into separate train detection sections (or if it is then these divisions are not used for determining following train separartion)
b) the trains themselves know where they are, usually pretty precisely
c) the decision of where a train should brake is not "built into" the design of the signalling system once-and-for-all, but calculated "on the fly" in real time, depending on where the train ahead is, the speed at which the following train is travelling, the gradient / speed profile, perhaps the weather conditions and train loading factor etc.
Hence the assessment is all about:
1) how accurately the position of the two trains is known (there is always an uncertainty which increases with the distance travelled from the last "absolute positio reference" - the train is considered to get longer (its "virtual occupancy" is increased to depict the "worst case" and is therefore always longer than its physical length)
2) how accurately the speed on train 2 is known
3) the various delays in the system (obviously any calculation using data from different sources has to acknowledge that the best available information is always somewhat out of date and therefore may have changed since)
4) how quickly the train itself will actually respond to a braking command,
5) tyhe required safety margin separation between the trains when the second approaches closely the first one.
The basic concept is that train 2 must always in the worst case be able to stop a small safety margin prior to hitting train 1. For maximum capacity, it should drive as fast as it can for as log as it can, until the very last minute when it decides that it really must start braking to avoid a collision. However this would not be good either for passenger comfort (very high "jerk rate") or indeed environmentally sensible use of energy, wear and tear etc so the ATO will "smooth" the driving and therefore ease off acceleration early, coast, apply brakes lightly to start with before braking more
There are a lot of calculations to do which is why computer modelling is needed; with a good signalling system the headway will be more dependent on issues of train performance than it would be the signalling.
It is primarily a matter of ensuring that the train position accuracy is good, there is low system latency (inerent time delays for cmmunications and processing etc.)
(15-11-2011, 04:36 AM)onestrangeday Wrote: Hi Peter:
One more question is that as for my company railway network, there are vitually no lineside signals at all as we use ATC system (ATP) for train control. I am wondering how the headway is calculated ???? since there are no Yellow or double yellow aspect...... in route signalling perspective there is only RED & GREEN aspect !!!
So I think aspect sequence diagram is no longer applicable. Am I correct ?
So do you know how the headway is calculated for a railway network that only use ATP system ?
