Since there seem to be a lot of railway-knowledgeable people here, I wonder if anyone can explain something I've heard on the London underground.

Whilst the train is stationary (and only stationary,) a very loud Dudududududud can be heard under the floor. Sounds a bit like an air compressor.

@grumpysmiffy it's an air compressor building up air in the brake pipe reservoir after a brake application - in this case, after the train has braked in order to come to a stop.

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