U.S. regulators are investigating why two New Jersey Transit trains carrying 1,600 passengers were stranded inside Amtrak’s 108-year-old Hudson River tunnel to Manhattan last week after apparently striking stray electrical equipment.

Though no injuries were reported and full service was restored in time for Monday’s morning rush hour, the incidents are still under review, according to Federal Rail Administration spokeswoman Desiree French. The agency conducts about 100 investigations annually, covering fewer than 2 percent of the accidents reported on average each year.

The breakdowns, occurring almost simultaneously on Sept. 7, underscore the need to build a second tunnel and renovate what is currently the only passageway to New York City on Amtrak’s Northeast Corridor, the nation’s busiest passenger route.

The tunnel’s concrete casing and electrical lines were damaged by Hurricane Sandy in 2012. Amtrak says the structure is safe, though increasingly unreliable, and has less than 20 years of service remaining.

President Donald Trump has resisted requests for the federal government to cover a large share of a new tunnel, part of Amtrak’s $30 billion Gateway project for New York City-area rail improvements. Hours before the trains broke down last week, the Gateway Program Development Corp. reaffirmed to the Federal Transit Administration a $5.55 billion local funding commitment and urged that the tunnel be a priority in the next federal budget.

“We must expedite the Hudson tunnel project to improve the reliability, resiliency and flexibility of the busiest 10-mile section of the Northeast Corridor,” Stephen Sigmund, a Gateway Development spokesman, said in a statement. “This weekend demonstrates, again, that there is no time to lose.”

New Jersey Transit referred questions to Amtrak. Beth Toll, a spokeswoman for the national passenger railroad, said the trouble involved the catenary, or overhead electrical equipment, and an investigation was continuing.

The trains broke down around 11:30 p.m. and were retrieved by rescue locomotives, according to New Jersey Transit.

Train 3886, heading to Pennsylvania Station in Manhattan with 1,100 people aboard, lost power in the two-track tunnel’s southern tube. The crew reported a downed wire and that a metal section of the overhead power apparatus had punctured the roof of the eighth car.

The second train, 3297, came to a stop in the northern tube as it headed to Bay Head from Penn Station with 500 passengers. Power was restored, but as the train emerged in Weehawken, it “struck an object that was hanging in the overhead wire” and stopped, according to an NJ Transit statement.

The Federal Railroad Administration typically issues investigation reports within a year.