Two Amtrak maintenance workers were killed Sunday when a passenger train bound for Washington plowed into a backhoe machine they were operating just south of Philadelphia.

The train’s engine derailed but remained upright, as the crash tossed about passengers in the two adjacent cars, sending 30 people and the train’s engineer to hospitals with minor injuries. It was the second Amtrak derailment in a little more than three weeks, coming after a midnight wreck in southwest Kansas that injured 32 passengers.

Amtrak said its scheduled trains would run in the Northeast corridor on Monday, but that riders should expect delays between Wilmington, Del., and Philadelphia.

Sunday’s derailment occurred minutes after Amtrak’s Palmetto train, with 330 passengers and seven crew members, departed Philadelphia’s 30th Street Station at 7:32 a.m. and gathered speed as it began to pass through Chester, Pa., about 18 minutes later.

Investigators from the National Transportation Safety Board and Federal Railroad Administration will want to know whether the train was on the correct track. If it was, they will turn to why crews using the rail-mounted backhoe machine were on what’s known as a “live” track.

U.S. Rep. Bob Brady (D-Pa.), whose district includes the site of the derailment, speculated that “there must have been some kind of communication failure.”

“Everybody should know when a train’s coming,” Brady said. “They’re on schedule. It ain’t hard to figure out.  If I’m working on the track, I wanna know when the train’s coming. It’s a straight track, you could probably see it.”

The two workers, who were not identified on Sunday, were the third and fourth Amtrak employees killed in crashes since 2000. Only 18 passengers have died in that period, eight of them in a 2015 wreck in Philadelphia. One of the two people was the operator of the backhoe, an NTSB spokesman said after arriving at the scene. The second person was reportedly the supervisor of the maintenance work.

The NTSB will remove the event data recorder and video from forward and inward facing cameras mounted in the engine cab, sending them to their laboratory in Washington for analysis.

“We will be looking at mechanical, operations, signal, track, human performance and survival factors,” said Ryan Frigo, the NTSB’s lead investigator. Frigo was unable to fully describe the size of the backhoe beyond “a piece of heavy equipment,” but it crushed in the front of the locomotive and created a spider-web pattern in the cab’s shatter-proof front window.

Ari Ne’eman, 28, of Silver Spring, Md., said he was napping in the second train car when the crash jolted him awake.

“I woke up and the train was shaking wildly back and forth,” he said. “There was a smell of smoke, there was a little bit of fire. And then the window across the aisle from me blew off the train.

“Probably the worst way to wake up from a nap,” he surmised.

Ne’eman, a disability advocate with the Autistic Self Advocacy Network in Washington, was heading back from a Friday speaking engagement at the United Nations autism awareness event in New York.

He said there was commotion as the train came to a bumpy halt, but conductors acted urgently to keep passengers safe. When a few passengers exited the train on their own, he said, a conductor shouted for them to come back aboard.

“He shouted very quickly and very actively that they had get back on right away and it was very dangerous,” he said. “At the time, those were still live rails.”

Ne’eman said passengers were ushered to the back of the train. Emergency personnel evaluated the injured, surveyed the train cars and handed out water. Passengers were taken to a local church and then driven by bus to Philadelphia’s 30th Street Station, the city’s main rail hub.

Ne’eman stood in line shortly after noon Sunday, waiting for information on how to obtain his luggage and alternative travel arrangements.

Amtrak shut down service between Philadelphia and Wilmington for much of the day, resuming limited service by mid-afternoon Sunday.  The disruption caused havoc as travelers, many of whom were returning home from the weekend or preparing for the work week, scrambled to find alternatives.

John Rison of Richmond, Va., said: “Things went flying. There was smoke. People came into our car and were saying that the windows were blown out in some cars.”

A photo showed the car just behind the engine with broken windows and what appeared to be soot around them. There was no indication of what might have caused a fire.

Allyson Aborn, 68, a psychotherapist who lives just outside of New York City, was relaxing with her eyes closed in the second car as she headed toward Washington to visit a daughter-in-law in Bethesda, Md., who was expecting to give birth Sunday.

She suddenly she felt the train’s brakes slam on, throwing her into the seat in front of her.

“I saw a huge fireball outside of the window and smoke pouring in,” Aborn said. “In the car ahead of me, the conductor yelled for everyone to get down under their seats. But there was nobody in my car to yell that to us.”

Passengers filed out of the train. After being examined, she took a Lyft to Wilmington and was able to catch a train back to her home in Scarsdale, New York.

Meanwhile, her healthy new grandson was born.

“It’s a good ending to a crappy story,” said Aborn’s husband, Bruce Doninger.

The derailment was about 25 miles from the North Philadelphia location where a May 2015 derailment killed eight and injured more than 200.

The 2015 crash, occurring in the nation’s busiest rail corridor, prompted questions about the safety of the country’s aging rail infrastructure. The train in that incident sped up from 70 mph to more than 100 mph in less than a minute before derailing. Authorities are still investigating the exact cause.

It was one of the deadliest crashes ever to occur in the Northeast Corridor, the heavily trafficked Amtrak route stretching from Boston to Washington.

Freelancer Bobby Allyn contributed to this report from Chester, Pa.