Rail services in the south east of England - which rely on a third rail electrification system - were particularly badly disrupted.

BBC: Transport debate

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

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