Trains in between Northampton and both Rugby and London are being disrupted because of an obstruction on the track, National Rail has said.
Passengers at Northampton station were also told by an announcement that an object had struck a train.
London Midland services may be cancelled, delayed by up to 30 minutes or revised as a result, warned National Rail.
Tickets will be accepted on Virgin Trains between Milton Keynes Central and Birmingham New Street.
The disruption is expected to continue until 15:30.