Two people were hurt when a train derailed near Watford Junction after torrential rain caused a landslide at the mouth of a tunnel.
The 06:19 BST service from Milton Keynes to Euston left the track at about 07:00, Network Rail said.
Part of the train derailed and was clipped by another. Both trains remain in the tunnel after the fire service helped to evacuate about 384 people.
Two people were treated, one for a neck injury and the other for chest pain.
A Virgin train was sent to the scene to take stranded passengers to King's Langley.

London Midland 06:19 train from Milton Keynes to Euston has hit an landslip and derailed on the approach to Watford Tunnel.

It has come to a stand within the tunnel and has come into contact with the 06:39 LM Euston - Birmingham.