Rail passengers in the West Midlands suffered major delays in the rush hour because overnight engineering work on the West Coast Mainline overran.
Network Rail, which is doing the work, said equipment broke down early on Monday morning, so it could not warn commuters in advance.
All local services were cancelled between Coventry and Birmingham International, Central Trains said.
Buses were laid on between the stations until the line reopened at 1035 BST.
A spokeswoman for Central Trains said Network Rail could not give the route back to the company at 0900 BST as promised.
The upgrade of the West Coast Mainline, between London and Scotland, is costing £8bn and will, when completed, result in shorter journey times.