A fault on a Norfolk bridge means all lines that usually travel over it are blocked.
The swing bridge at Reedham is not working, Greater Anglia said, meaning trains cannot cross the River Yare.
The operator added services running through the station at Reedham will be cancelled until the issue is rectified.
Trains between Norwich and Lowestoft are affected.
It said: "Greater Anglia and Network Rail are sorry if your journey has been affected by this disruption.
"Advance purchase tickets holders will be allowed to travel on earlier or later trains if their booked service is cancelled.
"If your journey with us has been delayed by 15 minutes or more, you may be entitled to compensation for a portion of your fare."
Disruption is expected until around 4pm.
Specific service alterations can be found on the Greater Anglia website.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here