Two people were injured after a train derailed when it struck a landslip caused by torrential rain near Watford Junction.
There was major disruption to services after the 6.19 service from Milton Keynes to London Euston came off the track at around 7am at the Hunton Bridge Tunnel near Watford.
One man suffered a neck injury and a woman was treated for chest pains, said East of England Ambulance Service.
ITV News reporter Luke Hanrahan talks to passengers
A spokesman for Network Rail said a "small portion of the train became derailed but remained upright" after it "struck a landslip".
Passenger Ian Saunders tweeted that those onboard the derailed train were evacuated and moved to another train. He praised the quick response from the emergency services and London Midland staff.
Radio reporter Sarah Lowther who was onboard said there was a "Dunkirk Spirit".
Speaking to Share Radio, Ms Lowther said: "It was the first time I've actually flown on a train; when we came off the tracks I assumed the brace position.
"Everyone is looking after each other with water and sugar but we're worried about our driver."
Another train travelling in the opposite direction "gave a glancing blow" to the derailed train, Network Rail said.
ITV News deputy political editor, Chris Ship tweeted:
Francis Thomas from London Midland told BBC Radio 4's Today programme the driver of the train "reported hitting something" which caused the train to derail.
"We have got extra staff on site assisting those passengers (on the train) and that is our focus - to look after those passengers, make sure they are all right and get them to their final destinations."
Passengers travelling south were reporting on Twitter that they had been stuck on trains for over an hour.
London Midland are advising passengers to use alternative routes.
"Emergency services and Network Rail engineers are on site and passengers are being transferred to continue their journeys," he added.
"Initially, all lines of the West Coast main line at this point were blocked but two of the four lines reopened at about 8am and services have resumed."
A spokesperson said that there will be disruption to services in and out of London Euston for at least most of the morning.
Southern Rail confirmed that they would be accepting London Midland tickets.
A statement on the National Rail website said that "disruption is expected to continue until the end of the day".