Many branch lines use diesel engines and the Harlem-Hudson line uses a third rail system to draw power.

WSJ: Commuters warned: Summer heat may slow Metro-North

Trains between London and the South East of England were particularly affected with the third rail system being iced over.

BBC: Transport Committee

Mr Hammond told MPs that there was a possibility of a roll-out of a heated third rail system, describing it as "particularly vulnerable".

BBC: Transport Committee