"Sweating the rails" phenomenon consists in:

The phenomenon of "sweating rails" involves the condensation of water vapour from the air on the cold, metal surface of the rails. It occurs most often in autumn when, after a cool night, the air temperature rises faster than the temperature of the rails. The thin layer of moisture formed in this way creates a very slippery surface, which significantly reduces the grip of a vehicle's tyres when crossing the tramway.