At about 1:30 a.m., the axle on the rear car of a 7 train came loose and sent the car off the rails at the Willets Point station in Flushing.
There were passengers on board at the time, but no one was hurt.
Crews worked through the night and brought in a crane to help with the repair work.
The incident caused some subway delays but 7 train service into Manhattan was restored by 8 a.m.