Service will remain suspended along a major chunk of the Orange Line through April 11, one week longer than the initial plan, to allow for repairs in the wake of last week's derailment, the MBTA announced Thursday.
Buses will continue to replace trains between Oak Grove and Sullivan Square for what is now a projected four-week span.
A T spokesperson said adding another week will allow crews to perform additional work at Wellington station, where the March 16 derailment took place. Workers will replace the track crossover at the incident location with a "custom-built replacement" and will replace track on both the northbound and southbound sides of the station platform.
Investigators have not determined the cause of the derailment, which occurred at a low speed with about 100 passengers on board. The incident "significantly damaged" a track switch, which prompted the MBTA to shut down train service for repairs.
Crews are also targeting additional improvements during the offline period such as tie replacements at Malden, infrastructure repairs at Sullivan and station improvements at Oak Grove, the T said Thursday.
The train that derailed was one of the new Orange Line sets, and the rest of the new cars on both the Orange and Red Lines remain out of service while the T and manufacturer CRRC MA analyze whether a vehicle fault contributed to the derailment.