[UPDATE BELOW] The FDNY is responding to a fire and reports of smoke on the subway tracks at Union Square. A spokesman for the department tells us it's too soon to provide any details about the incident, but judging by this photo, the fire appears to be related to Con Ed.

A Con Ed spokesman confirmed a crew is at the scene, but could provide no further details yet. One person describes an ominous "rumbling," and eyewitnesses and the FDNY say there was smoke in the station—the FDNY spokesman told us they responded to call about "smoke from hand brakes on a train." But an MTA spokesman tells us, "We have nothing going on at Union Square." Reports on Twitter suggest otherwise:

14th Street by Broadway is closed due to the situation. We'll update as more information becomes available.

Update 2:30 p.m.: Con Ed spokesman Allan Drury says the fire is now out, and there were no injuries and no outages. The cause, according to Con Ed, was "failure of electrical cables." Asked about what might have caused the reports of rumbling and loud booms, Drury said he didn't know, but added that the fire was not caused by a transformer explosion.

Update 3 p.m.:
Drury called back to tell us that the booming and rumbling were attributable to the electrical fire.