Headway- explain meaning

[PJW]

Because the headway is the time (or distance) between two tratin; to compare like with like you must compare the same place on each train and conventionally this is the front. However the separation distance betweeen two trains is that betweeen the closest place on the two trains and therefore the offset between the front and the rear ins important- i.e. the train length.

The train length is often pretty insignificant percentage of the signal spacing and as such does not make a huge fifference; in other scenarios it can be extremely significant and can even be more than signal spacing in some circumstances.

Look at the Module 2 portion of the Study DVD or any description of headway

Can somebody give me an explanation on headway, and it's meaning please?
Many thanks.

In very simple terms, it is the minimum interval that one train can follow another (at the designed speed) without the following train receiving a cuationary aspect. For mainline three aspect signalling, this means that the back of the first train must just clear the overlap of a signal before the following train reaches the sighting point of the signal that will have just stepped up from yellow to green (ie it will be a distance of [two complete signal sections plus an overlap length, plus a sighting allowance, plus a train length] behind the first train).

Given that headway calculations form a significant part of the module 2 paper, such a short explanation cannot hope to have covered everything!

Hi, Why we are taking the train length here..?