Passengers are being advised to check their train times today (22 March), following disruption to services between Northampton, Milton Keynes and London Euston.
Trains may be delayed or cancelled until around lunchtime, following track problems at Milton Keynes earlier this morning.
The line was closed for a time for repair works to take place, and reopened at around 9.30am.
However, as trains and train crew are out of position as a result of the issue, it is expected to take a number of hours before a full timetable can resume.
A Network Rail spokesman said: “We are really sorry to train passengers impacted today by a track fault near Milton Keynes. Our engineers worked to fix the problem as soon as possible.
"Passengers are advised to check National Rail Enquiries or with their train operators for the very latest travel information.”
Important sensors on the track, which measure a train’s speed and location, were damaged near to Milton Keynes. Without the data the sensors provide, signallers cannot run trains safely on the network.
Rail replacement buses will remain in place to supplement train services until a full timetable has resumed. These are operating to all stations between Northampton and Bletchley, and also between Watford Junction and Milton Keynes Central.
Buses that had been operating to and from Wellingborough are no longer running due to a separate incident in the Irchester area.
London Northwestern Railway tickets will continue to be accepted on Virgin Trains, East Midlands Trains, Chiltern Railways, Thameslink services until further notice.
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Passengers delayed by 15 minutes or more by this disruption can claim compensation through Delay Repay