Buses are replacing trains on some late-night services between Norwich and the capital while overhead cables are replaced.
Until March 21 Network Rail engineers will be carrying out improvement works on Monday, Tuesday and Wednesday nights between Shenfield and Witham.
On these days after 10.30pm bus replacement services will run on this part of the journey.
The existing overhead cables are 60 years old, and the improvement work is part of Network Rail's �200m overhead line upgrade project.
Dave Ward, Network Rail route managing director, said: 'There is never an ideal time to carry out this scale of work and we thank passengers in advance for their patience. We do apologise for any disruption this may cause, but people will understand that the work is absolutely necessary.'
Network Rail is suspending all major engineering work while the Olympics and Paralympics are on, and the upgrade work between Shenfield and Witham will then resume on Monday, Tuesday and Wednesday nights between September 17 and October 24.
For more information visit www.nationalexpresseastanglia.com or www.nationalrailenquiries.co.uk
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