The main rail line between Norwich and London is operating normally for the first time in nearly two months.
The line had to be lowered by 18 inches
A rail tunnel at Ipswich was closed to allow the line to be lowered to accommodate larger freight containers.
Meanwhile passengers had to take buses between Ipswich and Manningtree, adding to journey times.
One Rail said tests were carried out on the new lines on Sunday to ensure the safety of the work. After the all-clear, the line opened Monday morning.
Peter Meades from One Rail said there had been positive feedback from passengers over the bus service between Ipswich and Manningtree, and the twice a day train service between Norwich and London via Cambridge.
He said: "Generally we can all be pleased at how it's gone, but part of that credit is down to our passengers.
"There will always be the need to maintain and carry out improvements and certainly this is a very major project completed on time and successfully."
The tunnel has been closed since Sunday, 11 July.
The line had to be lowered by 18 inches at a cost of £5m.