About 600 straphangers were evacuated from a Manhattan-bound C train Sunday morning after part of the train that connects to the third rail became detached, causing smoke in the subway tunnel, according to a Transit Workers Union official.

“I saw a small explosion or something. It looked like a firecracker, but it was way louder,” said Raj Alexander, who was in the last subway car.

She said she was thinking, “ ‘ Please don’t let it be an explosion.’ I mean, you never know, it’s almost the anniversary of 9/11.”

The faulty train was stopped in the underwater tunnel to Manhattan after leaving the High Street-Brooklyn Bridge station. Passengers were evacuated onto a second empty train, which then let the passengers out at High Street-Brooklyn Bridge, according to the MTA.

The smoke was caused by “a detached current collector, also known as a ‘shoe’, that connects train cars to the electrified third rail,” said MTA Communications Director Tim Minton.

“The cause of the detached shoe is under investigation,” Minton said.

The FDNY responded to the alert at around 10:30 a.m. after the detached “shoe” started to spark, which caused the smoke, according to the MTA.

At least one passenger was seen receiving oxygen while being assisted to street level by the FDNY.

Normal service was restored just after 1 p.m., the MTA said.

Additional Reporting by Oumou Fofama