Now through Friday, service is suspended on the Nos. 1, 2 and 3 lines between 34th Street-Penn Station and Atlantic Avenue in Brooklyn.
The lines are closed from 10 p.m. to 5 a.m.
The FASTRACK repairs began Monday.
The MTA also shut down service between the two stations in February, shortly after it began its FASTRACK program earlier this year.
By completely suspending train service over a segment of a subway line, MTA workers get uninterrupted access to tracks, signals, cables, lighting, third rail components and platform edges.
Four FASTRACK line shutdowns were completed in the first quarter of this year, which resulted in an unprecedented amount of work being accomplished, according to the MTA.
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