All passenger trains have been cancelled between Peterborough and Ely after a fault was found on a bridge.
Network Rail Engineers identified a problem on the structure near Manea in the Cambridgeshire Fens on Thursday.
Train services were suspended shortly before 07:00 BST on Friday and are not expected to be running before Saturday.
East Midlands, Greater Anglia and Cross Country trains are all affected and bus replacement services are operating while emergency repairs are made.
Network Rail said the fault was identified following a "regular survey of the bridge structure".
Engineers hope to be able to repair the fault in time for the weekend, a spokesman added.
Greater Anglia said ticket restrictions will be lifted during the disruption and all the train operating companies have "agreed a universal ticket acceptance".
East Midlands Trains said on its website its services on the Liverpool Lime Street/Nottingham/Norwich route were affected and a "specialist team" from Network Rail was expected to make "a full assessment of the bridge with a view to begin repairs at 10:00".
Contrary to earlier reports, Network Rail said the bridge had not been hit by a lorry.