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Clive D.W. Feather · September 15th 16, 10:52 PM posted to uk.railway,uk.transport.london

On 15/09/2016 16:23, wrote:
Very likely it means "Down Line", with "U" used if the train is being
switched to the Up Line.
I believe the theatre lamps on SPILL Down indicate an X when crossing over.

Quite possibly. X is the standard notation for running "wrong line".

The lines out of Euston are labelled A to E and every signal from Euston
to Camden Junction displays the letter of the line the train will be on
at the next signal, even if there's no choice.

I'm not sure that I understood that?

Each line between Euston and Camden is lettered A to E.

A train departing Euston will pass 5 or 6 signals, including the
platform starter, in that length. Every one of those signals has a
theatre indicator showing one of those letters. This includes
straight-ahead routes and, in particular, includes those signals that
have no facing points before the next signal.

Consider the following (simplified) layout:

|-O 101 |-O 105
----------------*---------------- line A
|-O 203 \ |-O 207
------------------*-------------- line B

In a traditional layout (say the lines into and out of London Bridge):

101 to 105: plain signal, no theatre or feather
101 to 207: number 4 feather or theatre "B"
203 to 207: plain signal, no theatre or feather

Euston-style layout:

101 to 105: theatre "A"
101 to 207: theatre "B"
203 to 207: theatre "B"