In message , Clive D. W. Feather
writes
In article , Steve Fitzgerald
] writes
Driver's knowledge is that only an automatic (running) signal (those
generally numbered Axxx) can be passed at danger in this manner.

On parts of the Northern Line this is Sxxx instead; there was a signal
box coded A once upon a time.

Then there is a further complication in that we also have things
called X-signals (usually numbered like WRX61, although we also have
X855, ie no IMR code)

Also FXD855, meaning Floodgate protecting X signal on the District. The
numbers are usually taken from the automatic signal numbering sequence
(that is, X855 or FXD855 would lie between A853 and A857).

- these are normal Automatic signals but it has been decided that they
must be treated as semi-automatic in the event of any failure. Just
to add more fun to this, an X-signal can have an illuminated 'A'
associated with it which then turns it back into a normal automatic
signal whilst the A is lit!

The reason for X signals can be something to do with the interlocking.
For example, IIRC there is an X signal approaching Kennington on the
southbound Northern Line. If you passed it towards the station, then
under some circumstances your train will approach lock the pointwork
for some period of time, something that isn't desirable. So, although
it doesn't protect anything specific like a junction, the signal is
made an X signal so that you don't pass it until the signallers have
everything set up.

I knew all that I was just trying to keep things simple!
--
Steve Fitzgerald has now left the building.
You will find him in London's Docklands, E16, UK
(please use the reply to address for email)