A YouTube video produced by the agency shows work being done on the mile-long Montague Tube, which connects Downtown Brooklyn with Lower Manhattan via the R train.
The 14-month project, which started at the beginning of August, is being done in two phases. First, workers will repair tracks, lighting, circuit breaker houses, power substations, pump rooms, the fan plant, power cables and ducts, the MTA said.
Next, workers will focus on repairing signal equipment.
The tunnel suffered extensive damage during the hurricane when it was flooded with saltwater, corroding equipment, the MTA said.
There is no R service between Court Street in Brooklyn and Whitehall Street in Manhattan during the project. Late-night N and weekend R service is being rerouted over the Manhattan bridge.
The project, which will cost an estimated $300 million and is being funded by federal Sandy relief money, is expected to be done by October 2014.