It happened on the D line, just south of the Yankee Stadium station in the Bronx.

Sources tell NY1 a 54-year-old maintenance supervisor tripped or fell while walking along the tracks and landed on the electrified third rail, suffering severe burns.

Andy Byford, the head of the MTA's bus and transit operations, visited Harlem Hospital, where the supervisor is being treated.

"In speaking with our colleague's wife, she did ask me - I asked her actually. What would you like me to say to New Yorkers, and she said, “can you just say that we appreciate the support" and to keep her husband in their prayers," Byford said.

The MTA says the supervisor, who has 12 years on the job, was overseeing a crew inspecting track ties at 3 a.m. Friday.

The line was in service, meaning the third rail was carrying more than 600 volts of electricity.

"That's not unusual, but obviously, what is unusual is that this incident occurred," Byford said. "It's very concerning to me."

A source tells NY1 that members of the supervisor's crew were about 50 feet away from him when they heard an explosion, looked up and saw his clothes on fire.

The crew members snuffed out the flames with their work vests and helped him climb through an emergency gate to get to an ambulance.

Byford declined to discuss details, saying the incident is under investigation.

But the incident highlights the dangers MTA track workers face in handling even the most routine assignments necessary to  keep the system maintained.

"Personally I worked the tracks myself at one point in my career. You try not to think too much about it," Mike Carrube, president of the Subway Surface Supervisors Association.

Carrube says the MTA has made strides in employee safety in the 30 years since he first worked in the subway.

But he said it is impossible to make the job 100 percent safe

"Transit workers each and every day go to work and they deal with the hazards, the dangers of the job," Carrube said.

A danger highlighted by an incident involving the third rail, early Friday morning.