Every LIRR branch was delayed and several trains canceled after debris struck the electrified third rail at Penn Station during Friday's evening rush hour, authorities said.
"A piece of debris from an unidentified train snagged the third rail," said Amtrak spokeswoman Christina Leeds. The debris was dragged along the rail, she said.
She said power went out at 4:55 p.m. and was restored at 5:54 p.m.
The tracks, maintained and controlled by Amtrak, are used by the national railroad, Long Island Rail Road and New Jersey Transit.
Leeds said the debris did not appear to come from an Amtrak train.
Late Friday night, investigators were trying to determine what exactly happened, from the type of debris that caused the damage to the train that carried it, authorities said.
Investigators did not know whether the rails affected were even the ones used by the train with the debris, Leeds said.
LIRR was notified at 5:07 p.m. that power was cut off after the mishap, said spokesman Aaron Donovan.
He said no LIRR train was involved and there were no reports of injuries.
New Jersey Transit spokesman John Durso Jr. said he did not know which railroad's train was responsible but that his agency was investigating with other railroads. He said service on his railroad was not affected.
Donovan said the train cars run close to the electrified third rail. If any part of the train comes in contact with the third rail or if the third rail is slightly out of alignment, he said, power may be disrupted.
The outage primarily affected LIRR's rush hour, Donovan said.
Eight trains from Penn Station to Long Island were canceled, and passengers were warned of Penn Station departure delays of up to 15 minutes, Donovan said.
By 10 p.m., service was back to normal on all lines, according to LIRR's website.