The collision between two trains on the MBTA's Green Line on Friday, in which one ran into the back of another, is such a rare occurrence that one transportation expert simply said it "doesn't happen."
Carl Berkowitz, a transportation and traffic engineering expert, told GBH News that most train accidents involve some sort of mechanical failure and occur at railroad crossings.
But Friday's incident is in a whole other ballpark.
"This shouldn't happen," he said.
Berkowitz said there's two possibilities as to what happened: mechanical failure or human error. He said that a total failure of the braking system was unlikely.
"There's the normal braking system, and on top of that they have track braking, where a device comes down on the track and some friction also slows down the trolley," he said. "So if this trolley was put into emergency braking and the normal braking failed, you still had the emergency braking."
Berkowitz believes the National Transportation Safety Board (NTSB), which is investigating the incident, will first look into whether the train operator was impaired by drugs or alcohol.
"And the second thing is they're gonna look into whether there's mechanical failure, whether or not he used his cellphone, whether there's any video information about the accident...but these accidents just don't occur," he said.
Twenty-five people, including the driver of the rear train, had to receive medical attention after the crash. That driver has been placed on administrative leave by the MBTA.
A spokesperson for the Carmen's Union, which includes MBTA subway car, trolley and bus operators, said it will be closely reviewing any results of the investigation.
Berkowitz said investigators will be focused on why the accident happened.
"And we want to make definitely sure that it doesn't happen again, but 99% of the time, these type of accidents are the result of human error," he said.
In a statement, a spokesperson for the NTSB said four NTSB investigators are on the scene for the investigation. The board does not expect to complete the full investigation for one to two years.
"At this early stage of an investigation, NTSB does not state a cause but will provide factual information when available," the statement read. "A preliminary report, which includes all the factual information available to date, is expected to be published in a few weeks."
