On 31 Mar, 23:27, "Clive D. W. Feather" c...@on-the-
train.demon.co.uk wrote:

They are telling the driver which route he's going to take (so he knows
which speed restrictions apply and which is the next signal to obey).

And normally known as 'theatre type route indicators'. Except that in
the report into an accident at Euston in 1949, the inspecting officer
describes them as 'a route indicator of the music hall type'.

Was this a generally used term at one time, or is the inspecting
officer simply confused?

The report is available on the Railways Archive site, at:

http://tinyurl.com/3xkk2n

The reference is in paragraph 2.