The incident happened near Milton Keynes Central station soon after 12.30pm today (June 26).
At this stage, it is not known how serious the incident is or if there have been any injuries.
Lines through Milton Keynes Central station have been closed, with services from London Euston cancelled or delayed by up to 90 minutes.
It is affecting northerly Avanti West Coast and London Northwestern Railway services for destinations across the midlands, north of England and Scotland.
A National Rail warning reads: “A train has derailed in the Milton Keynes Central area, closing some lines.
“Trains running through this station may be cancelled, delayed by up to 90 minutes or revised.
“Major disruption is expected until the end of the day.”
