MBTA Green Line service has been suspended on the B Branch because of a collision between two trolleys near Babcock Street that left dozens of people hurt, officials said.
None of the 25 injuries were thought to be life-threatening, according to the Boston Fire Department and Emergency Medical Service, which were two of the agencies at the scene of the crash on Commonwealth Avenue near Boston University.
Train service on the branch was suspended between Packard's Corner and Kenmore, according to the MBTA. Shuttle buses were replacing trolleys between Kenmore and Washington Street.
Both trains were heading westbound out of the city when one rear-ended the other around 6 p.m.
"The trailing train collided with the train in front of it," said MBTA General Manager Steve Poftak.
The MBTA says 65-70 people were on board both trains.
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It wasn't immediately clear what caused the crash, and Poftak didn't want to speculate as the investigation began.
"We will obviously get to the bottom of this. This should not happen and we will find out why it happened and will ensure that it won't happen again," he said Friday evening.
Poftak added that investigators were looking into what speed the train in back was going. The speed limit for the trains is only 10 mph in the spot where the crash took place.
"The only conclusion we can draw is obviously, at some point, they became too close together," said Poftak. "That's a situation that should not happen."
The MBTA's safety department, along with state and federal oversight authorities, will also look into whether this was operator error or some kind of mechanical malfunction, and what caused the two trains to bump so violently.
"Our first thoughts are with our passengers and our employees who were injured," Poftak said, adding that EMS said all the people injured were conscious and alert.
Footage from the scene showed two damaged trolleys stopped on the track near each other, and people were seen being loaded into ambulances. Multiple fire trucks and police cruisers were there as well, and Commonwealth Avenue was partially closed.
Poftak couldn't give a timeline for how long it would take for the investigation to move to the point where the trolleys could be put back on their tracks and moved, restoring service. He expected state transportation safety oversight workers to arrive by the end of the night, and the National Transportation Safety Board has been alerted as well.
The driver of the train that crashed into the other was among the people hospitalized, Poftak said. He couldn't say how fast the train was going, what the speed limit for that section of the track was or what theories are being investigated.
"There's a variety of possible causes, I really don't want to speculate," he said.
A man on board one of the trolleys said his was pushed from behind, with people inside thrown to the floor by the violent collision.
People nearby said the crash sounded like fireworks.
The trolleys are on the same side of the track, and were headed away from downtown Boston at the time of the crash.