Train passengers are facing disruption and service cancellations due to problems with signals.
Operators Greater Anglia and Great Northern said a fault with the signalling system near Ely meant some lines were blocked on Friday morning.
Greater Anglia services between Norwich and Cambridge and Ely and Norwich have been delayed.
The 7am Cambridge to Norwich has been delayed at Cambridge and is now expected to be 50 minutes late arriving in Norwich.
A spokesman said: “Train services running through Ely station may be cancelled, delayed or revised/. Disruption is expected until further notice.”
Great Northern said it had been unable to run early morning services between King’s Lynn and Ely, and some services between Ely and Cambridge had been cancelled.
The 8.44am from King’s Lynn to London has been delayed.
The operator tweeted: “We have requested buses, and these should be in place by 8am. If you can delay travel until later today, please do.”
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