Amtrak service along one of the busiest corridors in the Northeast was resuming early Monday with some delays after a deadly derailment in Pennsylvania.

Two Amtrak employees were killed Sunday morning when Amtrak train 89 hit some construction equipment -- a backhoe -- and partially derailed in Chester, some 20 miles south of Philadelphia. Nearly three dozen people from the train were taken to hospitals.

The train was traveling from New York to Savannah, Georgia with 330 passengers and seven crew members on board when it came to a sudden and deadly stop.

The two employees killed were working on the backhoe when it was struck, reports CBS News transportation correspondent Kris Van Cleave.

The National Transportation Safety Board is leading the investigation into the incident.

NTSB investigators have recovered the event data recorder. The forward-facing video and inward-facing video from the locomotive have been send to their labs in Washington, D.C.

They hope some light will be shed on what exactly happened.

What was the backhoe doing on the tracks?

"As part of our investigation, we will be looking at the track structure and the work that was performed at the time of the accident," the NTSB's Ryan Frigo told reporters.

"We'll be looking at mechanical, operations, signal track, human performance and survival factors," he added.

The impact ripped a gash into the train's first passenger car.

Beth Blakely was riding in the car right behind it.

"It took a long time to stop. ... All you could see was a lot of dirt and debris flying past the train. And then there was a couple of fire flashes. And then we came to a stop," she recalled.

Steve Forbes, chairman and editor in chief of Forbes Media, was on the train and tweeted, "I'm fine, prayers to families of those killed and injured."

This incident comes less than a year after 8 people died and more than 200 were injured when Amtrak 188 derailed in Philadelphia. That train was headed from Washington, D.C. to New York. The cause is still under investigation. But the train was going two times the speed limit into a curve.

With this new crash, investigators have another puzzle to solve.