What Is The Purpose of This Mysterious Gauge On The Underground?

I wrote this post today on my way to the hospital with my son, Henry. I’ve taken the day off to be with him while he undergoes major dental surgery.

Henry is a very observant boy and noticed the gauge that’s pictured here under one of the seats on the train. We watched it and it fluctuated randomly, but seemed to go up when the train was stationary and down when it was moving.

The Internet is a great way to tap into the pooled intelligence of its readers. So help me answer my son’s question—what is this mysterious gauge for? I wonder if it’s some kind of current measure or something?

UPDATE
I have had three replies with some overlap:

1. I believe the gauge measures the air pressure to the brakes; hence, when stationary, high pressure to keep the brakes on and low pressure to keep the brakes off, although this doesn’t make sense because, if you have pressure failure, you can’t stop the train!! But it is a pressure gauge.
2. I think it is the brake pressure gauge.
3. I don’t know whether a professional out there can help you, or if you’re getting loads of opinions right now, but my theory has always been that it measures the pressure (were there any units mentioned on it?) on the coupling between the adjacent carriage; therefore, it fluctuates one way when braking hard and the other way when accelerating. It should be at a value in the middle when stopped, and moves about when traveling because of the unevenness of the ride. Just a guess from my time growing up in London and wondering the same thing a few times.