Network Rail said a major cable fire on Kilburn railway viaduct is going to impact services between Birmingham and London Marylebone for the rest of today (4 October).
The fire, reported at 4am, destroyed data and power cables taking out all the signalling - the railway equivalent of traffic lights - between Marylebone and Neasden Junction.
At 8am today Network Rail engineers were still waiting for fire fighters to finish damping down the blaze so they can get on to the viaduct to begin repairs, which are likely to take at least 24 hours.
Chiltern train passengers who had planned to travel from Marylebone today are advised is to use another station on another route. Chiltern tickets are being accepted by Virgin, London Midland, CrossCountry and GWR.
Martin Ball, area director for Network Rail, said:
At this stage it is not clear what caused the fire.