Trains between Norwich and London Liverpool Street are returning to normal after a fault with the signalling system at Shenfield.
Services were able to run through Shenfield but are doing so at a reduced speed and Greater Anglia advised passengers to travel via Cambridge and Ely.
The fault has now been resolved, though delays are still expected.
Greater Anglia and Network Rail apologised for the disruption and advance purchase tickets holders could travel on earlier or later trains.
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