In April, a power outage backed up trains around New York City and closed a key Manhattan station for 12 hours.
The Fire Department of NY said three patients were being treated for minor injuries.
Video shows scared passengers being escorted out of a trapped subway carriage following the derailment as hundreds had to be evacuated.
Passengers on one train tell WABC that it felt like it jumped off the tracks at around 125th Street.
"We started seeing sparks through the windows". Initially people panicked a little bit then and we realized we had to move. The cause was under investigation. It was not immediately clear why the brakes were activated.
Emergency personnel have responded to a Manhattan subway station where passengers were reporting smoke and a loss of power.
"This, to the best of my knowledge, does not look like a failure on the part of equipment, does not look like a failure on the part of the track itself", Lhota said.
New York's Metropolitan Transportation Authority says it expects to resume limited service on the A, B, and D lines following a subway derailment in Harlem.
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"During the whole time, it was just like 'What's going on? When are we going to realize how bad this crisis is and finally something about it", Gianaris said Tuesday.
The fire and police departments conducted a joint operation - involving 300 responders - in order to round up the passengers and bring them above ground, Nigro said.
Passengers ended up walking through the darkened cars using their cellphone lights and exiting onto the platform.
The footage shows firefighters helping people off the train and down the tracks. "Shortly after, we smelled a strong gas smell".
MacKrell then tweeted about the evacuation, saying: 'Folks calmed quickly and within 5 minutes MTA personelle were clearing passengers forward onto the 125 platform.
Gov. Andrew Cuomo called the derailment "an unacceptable manifestation of the system's current state".
The Riders Alliance, an organization of subway and bus riders in NY, blamed the derailment on the state's governor, Andrew Cuomo.