A landslip shut part of the rail line between Birmingham and London Marylebone.
Chiltern Railways said the landslip was reported at about 10:45 GMT near Harbury Tunnel in Warwickshire.
Buses are running between Leamington Spa and Banbury.
Network Rail said one of the lines had reopened, although there were significant delays. Engineers are still working working on the second line.
Chiltern Railways said tickets were being accepted on Virgin Trains in both directions between London Euston and Birmingham as an alternative route.
A spokesperson for the rail operator said there was no indication when services would return to normal.
Buses are also in operation between Stratford upon Avon and Banbury, while Cross Country is running buses between Coventry, Leamington Spa and Banbury.