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I noticed this pressure gauge on the Piccadilly Line of London Underground yesterday. It was located on the side of a row of seats.

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Close-up of the gauge at hand:

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click for larger version of image

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?

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This appears to be the pressure gauge measuring the brake pressure of the brake cylinders.

According to Transport For London,

The gauges that the requestor is describing are the Brake Cylinder Pressure Gauges that we have on every car.

These indicate the air pressure in the brake cylinders in real-time and are used by operational and maintenance staff for monitoring status and fault-finding.

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