Roland Perry wrote in message o.uk...
In message , at 23:54:01 on Wed, 4 Aug
2004, Mark Townend remarked:
I can remember standing on the westbound Central Line platform at
Liverpool St in the morning rush hour in the mid-80's, and you could
routinely see the front of one train entering the station at the same
time you could still see the rear of the previous one departing.
--

So illustrating platform reoccupation time as the fundamental limit on
capacity.

Consider the length of the platform as the braking distance for a train
entering the station at 'station entry' speed then the back of the train in
front must have just left the station (plus some additional 'overlap'
distance - see below).

The Central Line trains I observed were entering the platform at a
relatively slow speed. Perhaps 10mph. Common sense, if not signalling
practice, means that they could get within a couple of carriage-lengths
of the back of a departing train without any ill effects (although they
were perhaps 3/4 of a train away). After all, that's just what busses do
at every single busy bus stop, and they rarely rear-end one another.

It's only trains that have this concept of needing enough of a gap to be
able to stop blind from full speed.

Being able to key by a signal at danger (and proceed at caution)
allowed the NYC subway to run some very frequent services.
Unfortunately, a few years ago, there was indeed an accident which has
led to this practice being abandoned. What happened was that a train
was following another train over the Williamsburg Bridge, which is a
suspension bridge and consequently has quite severe grades at either
end. The second train reached the crest of the bridge and started to
coast down. Meanwhile the other train stopped and the second train's
brakes weren't good enough to stop on the downgrade. Result:
futtbucked. I still think that the abandonment of keying by was a
mistake - the practice should just have been restricted a bit better
to avoid such close following on downgrades.