In article . com,
TheOneKEA writes
[*] There are some specific exceptions, such as Turnham Green
on the Piccadilly, and the signalling in these places is altered
accordingly.
In the case of Turnham Green, both lines do have home signals - the EB
line's homes are A631^A and A631^B, and the WB's are A630^A and A630^B.
Would the overlaps on these homes simply be longer than usual, to allow
a train to alternately stop or pass through at linespeed?

Yes. Just as with a signal between stations. The overriding principle is
that a train stop hit at line speed should stop the train before the
point of danger.

[+] In general, a signal can only turn green if a train hitting the
train stop of the following red signal *at line speed* will be
stopped before the point of actual danger (e.g. another train).

This doesn't make much sense. Are you saying that if a train passes a
red signal at linespeed or higher and gets tripped, the signal in rear
could change to green if the entire train manages to exit that signal's
overlap?

Yes. But, in that case, the situation will still be protected. Um, let's
see:

|-O 1 |-O 2 |-O 3 |-O 4 |-O 5
-+---A---+---B---+---C---+---D---+---E---+---F---+---G---+---H---+---I-

Let's assume that braking distance from line speed is 1.4 times the
signal spacing. So:

1 is red if A, B, C, D, or E is occupied
2 is red if C, D, E, F, or G is occupied
3 is red if E, F, G, H, or I is occupied

etc. Suppose there's an obstruction at F. Signal 3 will be red because
it's within its block. Signal 2 will be red because a train tripped at
signal 3 won't stop until somewhere in G. Signal 1 can be green because
a train tripped at signal 2 from line speed will stop somewhere in E.

Now suppose a train runs past signal 1 at well over line speed and hits
the trip at signal 2. Signal 1 will be red at this point because the
train is occupying B and C. The train brakes but, because it was
speeding, it doesn't stop until somewhere in F. As the rear of the train
passes the E-F boundary signal 1 will revert to green *but* the
situation is still protected by signal 2 at red.

Clear? Or have I answered the wrong question?

--
Clive D.W. Feather | Home:
Tel: +44 20 8495 6138 (work) | Web: http://www.davros.org
Fax: +44 870 051 9937 | Work:
Please reply to the Reply-To address, which is: