Network Rail has apologised for the train delays for commuters heading into and out of London from Hertfordshire, Cambridgeshire and Norfolk on Tuesday morning.
The company was updating signalling equipment over the Bank Holiday weekend but it didn't work as expected after it was tested at the end of the work.
Hundreds of trains were cancelled or ran late and the disruption continued for much of the day.
Tony Raine from Network Rail said there would be a reduced timetable for the evening rush hour on Tuesday but services would return to normal on Wednesday.
Click below to watch the interview with Network Rail's Tony Raine