Two people have been hurt after a train derailed near Watford Junction after striking a landslide caused by torrential rain.
The London Midland service travelling from Milton Keynes to London Euston came off the track at around 7am this morning.
Another train travelling in the opposite direction "gave a glancing blow" to the derailed train, Network Rail said.
Ambulance Service said a man was treated for a neck injury and a woman for chest pains.
Their conditions were not thought to be serious or life-threatening.
The incident has caused major delays to London Midland services and passengers have been told to avoid travelling to or from London Euston.
Engineers remain on site and train services are now running through the area but it will be some time before a normal timetable resumes.
Network Rail say their priority is to make the landslip safe and fully reopen the railway as soon as it is safe to do so.