Bakerloo purple signal

Today whilst waiting at Baker Street, I noticed a purple lamp light up right at the start of the running tunnel. It seemed to come on briefly as the train entered the far end of the platform, and was extinguished before it came to a halt.
The lamp itself looked like a single aspect LU style signal. It seemed to work independently of the red/green signal.
What does it mean?

There's a tripcock tester at Baker Street. It might be to do with that.

It is indeed a tripcock tester. The light, which is “lunar white”, will illuminate as a train enters the platform. If the train passes the tripcock test it will extinguish; if not, it remains illuminated and the driver must contact the service controller.

Tripcock testers are provided after reversal or entry/exit to/from a line/sidings with a view to ensuring that tripcocks are tested at least once each trip.

Thanks gents. How does a tripcock tester test without tripping the cock, so to speak?

I believe that the ramp on a tripcock tester is fairly lightly spring-loaded, so the tripcock will push the ramp down completing a circuit which extinguishes the indicator light (thus proving it is in the correct position). But as it's nowhere near the impact of hitting a raised train stop, it's not sufficient to actually trip the train.

The tester also determines out-of-gauge, too long, too short and possibly missing tripcocks

Very interesting. Might see if I can spot the tester next time I'm that way.

It's a greased ramp with a spring underneath. The Bakerloo has them both roads at Baker Street as well as (if memory serves, and going back a bit) at Harrow & Wealdstone on the up and approaching Queens Park on the DC lines from Kilburn High Road on the down. That way they could guarantee tripcocks were tested on Queens Park - Harrow shuttles as well as the tunnel section.

Vague memories of some issues years ago surrounding planned Queens Park - Paddington booked reversers during engineering work where there was no facility to test the tripcocks. In the end I think fitters were physically testing them in the north sheds at Queens Park.