Delays were reported throughout the subway system, which has been plagued by problems this year.
One of the passengers on the train, Medina Browne, said: "At first we didn't know what was happening so it was just really an irritant, just an inconvenience".
While Lhota told reporters that seven people sustained minor injuries during Tuesday's derailment, the FDNY supplied a much higher number.
One rider noted that the A train broke down and "caught fire" at 125th Street.
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The Metropolitan Transit Authority confirmed on Twitter that a subway derailed at 125th street and that emergency responders were on scene.
They also reported breathing in smoke, and seeing sparks.
Three other trains approaching the station halted in their tracks.
It is believed two cars dislodged from the tracks due to a power outage, police said. Lhota said the emergency braking system on the train triggered, but it wasn't immediately clear why. Lhota said the smoke was caused by sparks and the garbage and sanitation along the tracks.
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A, B, C and D train service was suspended as a result, the agency said.
A student journalist tweets there was a train derailment near 125th. "It wasn't heavy smoke, but it was frightening", he said.
Gov. Andrew Cuomo has said he will give $1 million "Genius" grants to the three people with the best solutions for fixing the subway system's problems, some of which, like an aging signal system, are directly connected to delays. Emergency crews shut off track power after derailments to prevent evacuees from being electrocuted. Fire officials estimate that as many as 500 of the 800 people stuck on trains after the power outage exited the train into the tunnel on their own. Almost three dozen people were injured, the New York Daily News reported.
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