Rush-hour commuters suffered major disruption as no trains were able to operate between Milton Keynes and London Euston due to a fire.
Disruption to rail services on the West Coast Main Line was expected to continue throughout Friday and will also affect travellers from Northampton.
More than 50 firefighters were sent to tackle the blaze at a timber yard in Harrow, north-west London.
A Network Rail spokeswoman said: "A fire overnight at a timber yard in Harrow has damaged a trackside retaining wall.
"It will need to be dismantled, made safe and rebuilt. This is a big job expected to take all day.
Virgin Trains has lifted ticket restrictions meaning passengers can use services on alternative routes or travel on Saturday.
A spokesman said: "We are sorry for the inevitable disruption and delays to journeys."
Passengers on a London Midland service from London Euston to Milton Keynes were evacuated from a train at Headstone Lane.
The cause of the fire is not yet known.