Train services between Norwich and London are being cancelled or delayed after overhead cables were brought down near Chelmsford.
Travellers from Norfolk and Suffolk are advised by National Express East Anglia to use services through Cambridge.
Engineers worked through the night to repair the cables but only one track is passable on the line at Ingatestone.
Jonathan Denby, from National Express, said he had no idea when the line would be fully functioning again.
"We still have one line blocked just south of Chelmsford as a result of the major damage to overhead lines at teatime on Monday.
"That means only one track is open, so for travellers from Norwich we can only operate at a reduced level.
"This is a half hour train shuttle service during the peak to Ipswich to connect with other forward services to London.
Many services affected
"It may be marginally faster for Norwich travellers to go via Cambridge," Mr Denby said.
"We have no estimate from Network Rail when the repairs will be completed but engineers have been working through the night.
"This is damage to a significant section of track and engineers are working as fast as they can to repair the cables and when they do we can push the trains through."
The overhead wires have been damaged between Chelmsford and Shenfield and it is hoped repairs will be completed for a normal service on Wednesday 11 June, National Express East Anglia have said.
Many services are affected in Essex and train shuttle services will operate between Harwich and Colchester and between Braintree and Witham.
Lowestoft and Peterborough services will start and terminate at Ipswich.
First Capital Connect will accept National Express ticket holders.
A normal service will operate on the Metro/Southend and Southminster lines.
Keith Lumby from Network Rail said about a mile of cables had been brought down but the repairs should be completed by Tuesday night.
"An investigation was under way and Network Rail was determined to get to the bottom of the problem," he said.
The Norwich to London line had seen a number of incidents of overhead power cables being brought down by trains in the past few years.