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That was good. From that thread

” However, spring brakes are used as a parking brake on modern trains. In the the simple air and vacuum brake systems, when a train was parked out of service for any length of time, the crew had to remember to apply the hand brakes (screwing it down), otherwise the air would sooner or later leak out of the system and allow the brakes to release. Modern trains have auxiliary brake cylinders with powerful springs to apply the brakes. Whilst the train is ‘cut in’, with the compressors running, compressed air is supplied to these auxiliary cylinders to hold the brakes off. When the train is parked and ‘cut out’, the air eventually leaks away, allowing the springs to apply the parking brakes. These auxiliary brakes are only sufficient to hold the train stationary on any gradient, and are not used in service. Source(s): Retired UK Train Driver”