In article , Spyke
writes
Just a quick question - does anyone know what a small illuminated 'J' might
be used for on the northbound met platform at Finchley Road station.

I'm guessing it's probably an indicator to inform a Met driver and/or
the station staff that a Jubilee line train is approaching, and that it
might be a good idea to let the Met train wait to allow passengers to
from the Jubilee line to transfer to it.

Correct.

There's also one on the northbound at Wembley Park, for the same reason.

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