I noticed this pressure gauge on the Piccadilly Line of London Underground yesterday. It was located on the side of a row of seats.
Close-up of the gauge at hand:
The pressure gauge seemed to randomly fluctuate between 0.5 and 1.5 bar when the train was in motion, with no significant correlation with the acceleration or the speed of the train. However, when the train was in stations and the door opened, the gauge reads out as ~3 bar. The close-up was taken when the train was stopped at a station.
What is this pressure gauge measuring, and why is it helpful to have it displayed in such a prominent location in direct sight?