It had been a quiet subway ride for Bryan Marden Saturday morning, his train jammed with passengers dressed for a game at Fenway Park, or like him, an afternoon at Boston’s annual Pride Parade.
But as their Green Line trolley rolled from Kenmore to Fenway, the routine trip was shattered when the train suddenly derailed. The trolley struck the side of a subway tunnel, the impact throwing helpless passengers to the floor, and then came to a sudden stop amid a cloud of dust and debris, according to Marden.
When it was over, 10 people, including the train’s operator, were taken to hospitals for treatment of their injuries. Some of the injuries were serious, authorities said, but they did not give details other than to say none were life threatening.
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“When the banging happened, people knew something was wrong,” said Marden, 19, who was not injured. “It was kind of chaotic.”
Officials Saturday were searching to find out what caused the single-car trolley carrying Marden and about 150 other passengers to derail around 11 a.m. on the outbound track of the D-Line branch, where the tracks split for the Riverside and Cleveland Circle trains, said Joe Pesaturo, spokesman for the Massachusetts Bay Transportation Authority.
The derailed train came to a halt hundreds of yards inside the tunnel, and Boston firefighters had to carry at least six injured people to a staging area at the Fenway MBTA stop, said Firefighter Brian Alkins, a Boston Fire Department spokesman. It took about 40 minutes to evacuate the train, he said.
Among the injured was the train’s operator, a 62-year-old man who was hired by the MBTA almost three years ago, Pesaturo said.
He was in stable condition Saturday afternoon, Pesaturo said, and was being treated for a leg injury, as well as some bumps and bruises.
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The derailment occurred before a double-header at Fenway Park, where the Red Sox were to play the Tampa Bay Rays beginning at 1:05 p.m., and the noon start of the 49th annual Boston Pride Parade, which drew thousands to the city Saturday.
Marden said it took about five or six seconds for the train to come to a halt after it derailed.
After it stopped, he and other passengers began checking on one another’s condition. Several people were not able to get up off the train’s floor, he said.
Lighting in the tunnel was activated, but dust kicked up by the derailment made it hard to see inside the train. Some people used their cellphones as flashlights, he said.
Marden said he pulled an emergency lever to open the train’s doors and went outside into the tunnel to look for stretchers to help move the injured, but found none. When he got outside, he said debris from the tunnel was scattered on the ground.
“After everything, I just wanted to make sure everyone was OK,” he said.
Emergency personnel were on the scene within minutes, Marden said.
When the train derailed, construction worker Antonio Carreiro was installing benches outside Fenway station when he heard a screeching sound.
People started coming out of the tunnel shortly afterward, he said, including one young woman with blood on her face lying on a stretcher, he said. Another woman who appeared “shaken up” was loaded into an ambulance.
It wasn’t until after passengers started emerging from the tunnel did he realize what happened.
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“It sounded like a pop, like fireworks,” he said. “We thought it was something for the parade, until we realized it wasn’t.”
After the train was stopped, its passengers, plus about 500 additional commuters on trains behind the stopped vehicle, were forced to evacuate through the tunnel, according to Steve Poftak, the MBTA’s general manager.
“Obviously, this is not the service we want to run,” Poftak told reporters at Fenway station Saturday afternoon. “Our first priority here at the MBTA is safety [and] we are going to conduct a full investigation into this derailment.”
On Saturday afternoon, shuttle buses replaced subway service between Kenmore and Cleveland Circle on the C-Line, as well as Kenmore and Fenway on the D-Line, the MBTA said on its website.
Regular train service resumed on those lines Sunday morning, the MBTA said.
Officials launched a comprehensive investigation that will look at whether there were any problems with the train, tracks, signals, electrical systems, or human error that led to the derailment, Poftak said.
The train’s operator will undergo drug and alcohol testing, which is standard procedure for any MBTA employee involved in a safety-related accident, Poftak said.
The train operator will be interviewed by investigators after he completes his medical treatment, Pesaturo said.
On social media, Marden and other passengers posted photos of the derailed Green Line train, which appeared to have struck the side of the subway tunnel.
No structural damage to the tunnel or overhead wire had been reported, Pesaturo said. Once the train was moved from the scene, the track and infrastructure will be inspected and any needed repairs will be completed.
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The last derailment on the Green Line occurred on Feb. 5, during the championship parade for the New England Patriots, Poftak said. That derailment was still under investigation, Poftak said.
There was no evidence to suggest a connection between Saturday’s incident and the one in February, according to Pesaturo.
Globe correspondent Lucas Phillips contributed to this report. Amanda Kaufman can be reached at amanda.kaufman@globe.com. Follow her on Twitter @amandakauf1. John Hilliard can be reached at john.hilliard@globe.com.