Rail passengers were warned of delays after all services in and out of Norwich were blocked this morning.
Greater Anglia said the severe disruption was due to a fault with the signalling system.
A spokesman for the firm said they were working "diligently" with Network Rail to restore the timetable "as soon as possible".
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A "very limited" number of rail replacement buses were operating between Norwich and affected routes and passengers were told tickets would be accepted on local bus routes between Norwich and Sheringham, Great Yarmouth and Lowestoft.
The lines reopened just after 7am but delays and disruptions continued until around 10am.
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