SHMD

because it helps to keep the engines cool of course!

From a driver on UK Rail Forums 'Pacer and Sprinter series trains have their heating supplied by waste engine heat by passing the coolant around the heating system for the saloons, this is 'topped up' by an Auxiliary (webasto) heater which burns diesel to supply heat and also a pre-heater for the engine in cold weather. The system is set to maintain a constant temperature - 20c if I recall rightly - and on Sprinters there is nothing to allow the crew to switch it off except for tripping the fans much which is frowned on'

Probably just the waste heat from the engine, if the control thermostat has failed and providing a way of 'venting' the engine heat in extremely hot weather to assist the unit completing it's journey.