BOSTON -- Federal transportation investigators have begun the difficult task of determining what caused a Massachusetts Bay Transportation Authority Green Line trolley to crash into the back end of a train ahead of it in Newton Wednesday, killing the train operator.

MBTA officials said the operator, identified as Terrese Edmonds, 24, was a part-time driver who had been employed with the 'T' since last July.

Investigators Probe Deadly MBTA Crash | 911 Call | MBTA Has Recent History Of Crashes

"This is a terrible tragedy and our hearts go out to the family of Ms. Edmunds and also our prayers for the speedy recovery of the others who were injured," Newton Mayor David Cohen said.

Ten other passengers on the two-car trains were injured in the collision, which occurred about 6 p.m near the Woodland Station stop. Both trolleys were headed outbound toward the Riverside stop, which is the end of the D line.

The Green Line, with more than 250,000 daily passengers, is the most heavily-used light rail system in the nation.

It took rescue crews more than seven hours to extract Edmunds' body from the mangled wreckage located near the Brae Burn County Club golf course. They spent hours painstakingly sawing through the metal of the crushed car to remove her remains, the firefighters briefly removing their helmets and bowing their heads in silence when they were finally able to locate the trapped victim.

"There's no speculation as to what might be the cause. We're going to leave that to the experts who come on the scene as we reconstruct what happened," MBTA General Manager Dan Grabauskas said.

What investigators do know is that there were roughly 200 people on board the two trains when the crash occurred. The first train had stopped, as required, at a red light ahead of the Woodland stop before starting its slow approach to the T stop. The second trolley plowed into it from behind, lifting both trains off the track and ripping the wheels from the second trolley.

"People were screaming, confused. I mean, there were fires on the sides. I looked quickly and there were people under the trains. It was a scary thought," passenger Barry Gallup said.

MBTA officials did not say how fast the train was traveling. Typically, they run no faster than 40 mph through the stretch of Newton where the crash occurred.

People in neighboring homes said they heard what sounded like a huge explosion and they ran through backyards to see what had happened.

"We use the services almost daily and I am just grateful that my family wasn't involved. But my heart is breaking for the families (of the victims)," neighbor Maria Arcese said.

EMTs treated five passengers at the scene. Six were taken to area hospitals and one was flown by helicopter to Boston Medical Center with serious injuries.

"It's a miracle that more people were not hurt," one neighbor said.

MBTA officials said commuters would be bused Thursday on shuttles between the Riverside and Newton Highlands stops while the track remains closed near the crash scene.

Shuttle buses will be stopping at Waban and Eliot stations to pick up passengers, then taking them to Newton Highlands for the remainder of the trolley ride into the city. Passengers were advised to leave at least 30 extra minutes for the commute. It was unclear how long the MBTA would have to continue shuttling passengers along that portion of the route.

Many early passengers Thursday were still trying to understand what had happened.

"I'm surprised it happened, especially with the track improvements that they did, that this happened. So, I don't know why, something like that ... especially when the train, the first one, was stopped," one passenger said as he prepared to board a shuttle at Riverside.

National Transportation Safety Board investigators will be on the scene for at least five days, but it may be a full year before the results of their probe into the cause of the crash are known.