Four transit workers were hospitalized early Thursday morning following what the MTA called a "minor" explosion during overnight work at the 14th Street-Union Square station.
The MTA says a cable made contact with the electrified third rail, setting off an electrical arc explosion at around 3:30 a.m.
The most seriously injured worker suffered severe but not life-threatening burns to his face, neck and legs.
The explosion occurred on the southbound express track of the Lexington Avenue line at the station, where crews were doing maintenance work.
The work was being carried out as part of the MTA's "Fastrack" program, which shuts down portions of subway lines for overnight repairs.
The head of the Transport Workers Union, John Samuelsen, says the accident highlights the hazards faced by transit workers.
"The job's incredibly dangerous. Anytime where you're working with iron and steel around a live third rail, industrial accidents are going to happen. We take precautions, we take many safety precautions. But ultimately, at the end of the day, industrial accidents will occur when you're dealing with this environment of iron and steel," Samuelsen said.
The most seriously injured of the victims is being treated at the burn center of New York Presbyterian Hospital.
The three other workers sustained minors injuries to their faces and arms.