Metro-North service is back to normal after a fire broke out Monday under the 138th Street Lift Bridge by the Harlem River.

The fire began before noon in pilings under the 138th street Lift Bridge by the Harlem River, temporarily stopping all service into and out of the city on the Hudson, Harlem, and New Haven lines.

Huge clouds of smoke billowed into the air and it took more than an hour until fire boats were finally able to knock down the flames.

New York City Fire Department officials say their biggest concern was the potential damage to the bridge above.

"If the fire gets hot enough, it can melt or substantially alter and affect the steel. It can also affect the conduits, the electric conduits that run underneath it, and that's what's being investigated right now," said FDNY Deputy Chief Paul Mannix.

The first fire trucks arrived within minutes, but it took more than a half an hour before crews were able to start fighting the blaze with fireboats.

"The challenge is getting water on the fire, getting a water supply. We don't have hydrants in this area because the FDR drive is so close," Mannix said.

A man who lives under the bridge with four or five other other people said he heard a huge explosion before the fire began.

"All of the sudden, I just heard a 'boom,' and then I saw the police looking down, saying, 'Get out of here," said eyewitness Luther Ridley. "And then two wires fell, and when the wires fell they hit the wood. And when it hit the wood, that's when the smoke came."

There are no reports of any injuries, and Metro-North authorities say no trains were involved in the fire.

After engineers inspected the bridge, all four tracks were put back in service just in time for the evening rush.

The cause of the fire remains under investigation.