Authorities still don't know why a New York City Metro-North train derailed on Sunday, killing passengers for the first time in the line's 31-year history.

Officials began investigating Monday morning while righting the seven toppled cars, which derailed while rounding a riverside curve on the Hudson line. Four people were killed, and more than 60 others were injured, The Associated Press reported.

MTA Transit is providing shuttle buses to service the 26,000 weekday riders that typically use the line. Around 150 people were on board the train when it derailed, which is about half of its capacity.

"We'd like to get service up toward the end of the week," Gov. Andrew Cuomo said.

The derailed train's locomotive was lifted back onto the track by crane around 4:20 a.m. Monday, while two cranes were hoisting the tilted car connected to it, according to railroad spokesman Aaron Donovan.

While the reason for derailment is still unclear, possible factors are speed, equipment failure, operator failure or a track problem, Cuomo said.

"It was actually much worse than it looked," he said.

"As the cars were skidding across the ground, they were actually picking up a lot of debris, a lot of dirt and stones and tree limbs were going through the cars so it actually looked worse up close," he said, adding that it was "your worst nightmare."

The train's engineer was also among the injured and hasn't yet been questioned as part of the investigation, NTSB board member Earl Weener said Sunday.

Officials don't know how fast the train was going, but the speed limit on the curve is 30 mph.

The four victims have been identified as Donna L. Smith, 54, of Newburgh; James G. Lovell, 58, of Cold Spring; James M. Ferrari, 59, of Montrose; and Ahn Kisook, 35, of Queens. Their autopsies were scheduled for Monday, according to the AP.

The National Transportation Safety Board  said it could take up to 10 days for its investigators to determine the cause of the incident. The board has been pushing railroads for decades to implement safety technology.

After Congress passed a rail-safety law in 2008, railroads were given until 2015 to install systems known as positive train control. Due to the expense and complication of the systems, officials have been trying to push back the deadline another five to seven years. In the "automatic train control" signal system, technology would automatically apply the brakes if an engineer fails to respond to an alert that indicates excessive speed, according to the AP.

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