Federal investigators arrived Thursday at the tracks outside Boston where two commuter trains collided and derailed during rush hour a day earlier, trapping and killing a train operator and injuring more than a dozen passengers.

Officials from the National Transportation Safety Board, including board member Kitty Higgins, were trying to determine what caused Wednesday's above-ground crash, which killed 24-year-old Terrese Edmonds near a station in suburban Newton.

Among the possible causes routinely investigated by the board are equipment failure, operator error and systems malfunction. A full report is not expected for up to 18 months, but an initial briefing was scheduled for midday Thursday, board spokesman Peter Knudson said.

The crash came hours after an elevated train derailed in Chicago, sending 14 people to hospitals. Officials there quickly blamed human error by the operator.

The two-car train Edmonds was operating struck the back of another two-car train approaching Woodland Station during the evening rush hour, said Joe Pesaturo, a spokesman for the Massachusetts Bay Transportation Authority. The trains had about 200 passengers combined.

"The first one was stopped at a red signal and was ready to proceed to the station when it was struck," he said.

The MBTA was busing passengers around the crash site Thursday, Pesaturo said. The train struck from behind had been removed from the scene, but the second remained as the investigation began.

"They need to thoroughly examine the second train before it is removed," Pesaturo said.

For several hours Wednesday night, firefighters struggled to free Edmonds from the mangled wreckage. She was finally extricated early Thursday.

"It is my unfortunate duty to report the death of one of our employees," MBTA General Manager Daniel Grabauskas said, adding that it was a "miracle" that there weren't more fatalities.

One passenger was in stable condition in a Boston hospital Thursday, Pesaturo said. Nine others were treated at Newton-Wellesley Hospital and about five were treated at the scene, he said.

In Chicago, authorities said a train operator apparently made two key errors in quick succession to cause a derailment that left passengers perched more than 20 feet above the ground.

"Everybody was screaming and hollering and you know, and praying for God," said 35-year-old Willie Jackson, who was aboard the train's second car when it derailed and leaned west off the tracks.

The operator failed to heed a red signal ordering him to stop, Chicago Transit Authority spokeswoman Noelle Gaffney said. After the four-car train went through the signal, it automatically activated a trip, which stopped the train.

But the operator moved the train forward again to a spot where the tracks weren't aligned, causing the rear end of the front car and the second car to derail but remain standing, with the other two cars still on the tracks, Gaffney said.

"He was going on the wrong tracks, or started to," she said.

Gaffney said there was still a possibility the aging transit system played a role in the derailment.

The operator, who has 31 years' experience, was cooperating with the investigation and will not be allowed to return to work until the probe is done, she said.

A total of 25 people were on the train, including one CTA employee. Some of the injured were put in ladder baskets and lowered to the ground, where they were put in ambulances. Others were led off the tracks via a nearby stairwell, officials said.