The line had been closed to electrify the route over a phased eight-month period. It is part of NR’s Railway Upgrade Plan, which was announced in June 2016, and work was due for completion by the end of February.
However, NR says that a number of structures which were built to carry the overhead lines were incorrectly designed and could not be installed at the planned locations along the 22.5km route.
Due to NR’s shortcomings, the line will be closed over a series of weekends and will likely involve another full closure later in the year.
The work, however, will not have an impact on the reopening of the line to diesel trains on February 27.
Bombardier is currently building 45 four-car class 710 EMUs to operate along the route from early 2018, doubling capacity and replacing existing two car DMUs.
The project also involves lowering five sections of track between Walthamstow Queens Road and Gospel Oak stations, to increase space for overhead catenary. Ten bridges have been reconstructed or strengthened along the route, and three electrical switching stations have been completed.
The project is being funded by the Department for Transport (DfT) and Transport for London (TfL).