A cracked rail on the line at Hemel Hempstead is causing cancellations and delays in and out of London Euston.
In a series of Tweets, London Midland have updated commuters on the situation after the problem surfaced during rush hour earlier this morning.
Several trains have been cancelled and others delayed after the fast line from London Euston and Milton Keynes was closed to allow engineers to fix the issue.
In another Tweet, London Midland told customers that cross ticket acceptance is now in place with Virgin Trains on services between London and Birmingham.
Trains running between Milton Keynes and Euston will be using the slow line, so congestion is expected.
Anyone making the journey to London from Bushey and Harrow this morning are advised to use the London Overground.