Fig wrote:
Possibly a tripcock tester?
http://www.trainweb.org/tubeprune/Si...photos.htm#Fig 3

The tester is usualy next to the track 3/4 of the way along the platform,
with the white light next to the starting signal.

That was my first though when I read the question. Tripcock tester
lights are often purple, but they are also white at times. The only
other option I can think of off the top of my head is its a platform
reverser signal, which indicates to the driver he is to turn on the
platform.