A train has derailed near Watford Junction station after striking a landslip caused by torrential rain, Network Rail said.
There were no reports of any injuries after the 6.19 London Midland service from Milton Keynes to London Euston came off the track at around 7am.
Another train travelling in the opposite direction "gave a glancing blow" to the derailed train, Network Rail (NR) said.
The incident caused major delays to services in and out of London Euston.
Martin Frobisher, route managing director for Network Rail, said a train hit a landslip caused by torrential rain, resulting in a small section of the train leaving the tracks a few miles north of Watford.
The train remained upright and there are no reported passenger injuries, he added.
"Engineers are on site and train services are now running through the area but it will be some time before a normal timetable resumes," he said.
Initially, all four lines of the West Cost main line were closed at the scene, but two were reopened at around 8am.
Mr Frobisher added: "Our priority is to fully reopen the railway as soon as it is safe to do so. A full investigation into what happened will take place."
Radio reporter Sarah Lowther, who was on the derailed train, said two trains were "kissing each other" in a tunnel.
She spoke of a "Dunkirk spirit" on board as passengers helped one another, but said she was "worried" about the driver, who had a bad back after the crash.
Speaking to Morning Money radio, she said: "The trees were taken down from the side of the rail line last year. Trees have roots, roots hold the mud ... The mud had nothing to cling on to.
"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."
A pregnant woman was on board who emergency services were trying to remove from the train, she added.
Ms Lowther got on the train at 6.47am at Berkhamsted, which was hit by an "oncoming" train at 7.01am.