Get On, Clip In, Fall Off

Public Service Announcement

I just want to point something out after having heard overheard a few other passengers making comments.

If you're sitting on a stationary or slow-moving train and a revving - or sometimes thumping - noise starts up under your feet, it's just the compressor starting up to keep the brake systems (which usually rely on air pressure to hold the brakes off) charged up with compressed air. Stopping the train generally means venting air from the brake system to the atmosphere, with the result that every so often the system needs to recharge itself so there's enough pressure to take the brakes off again.

It is not the sound of the driver "trying to get the train going again", nor the sound of the driver "revving the engines". Most importantly, it doesn't indicate that there's a fault with the train, unless it keeps going for a long, long time when it might just be an indication of an air leak somewhere. Because electric trains are more or less silent when they're stationary this is when you're most likely to notice the compressors running - when the train's moving, or if it's a diesel, there's often so much other noise that you won't notice the compressors starting up unless you're listening for them.

So now you know, and can make yourself look like a nerdy trainspotting smartass next time this happens by informing your fellow passengers that "Ah! The compressors have started up! The brake reservoirs must need recharging.".