Commuter trains? I believe that was an MBTA Subway line and not an MBTA Commuter Rail line.
It was two D-Green line trains that collided, basically light rail trains. And the media gets everything wrong, as every story I've seen has the conductor driving the train and being killed.
I've always been told the GREEN LINE is a tramway (or trolley), while the BLUE/ORANGE/RED are subways.
The PURPLE LINE is the commuter rail.
I was told this by the museum operator at the SEASHORE TROLLEY MUSEUM. Apparently, the largest museum in the world. There is, apparently a difference between a Tramway, light rail, subway, and train.
That said, the GREEN LINE train frightens me the most. It is loud, jerky, and uncomfortable. Apparently, it is also the business in the world? Which apparently prevents any sort of updates I'm told.