It's not just the MBTA that has been running push-pulls for a long time, so has every other US operator of locomotive-hauled commuter trains, and on the whole the safety record of push-pulls has been excellent. There have been cases where cab-first operation caused a problem. In the mid-1990s a MARC train running control car first ran a signal near Silver Spring, Maryland, and hit Amtrak's "Capitol Limited". In the resulting fire the engineer and several passengers who were riding in the cab car died. And there was also the recent Metralink incident outside Los Angeles when a car parked on a track caused a cab car first train to detail in the path of a second train, causing numerous fatalities. In both cases, had the affected train been running locomotive-first, it might have been better able to absorb the collision.
But those are rare exceptions to a general pattern. Given all the push-pull trains running in this country, and for that matter all the MU sets that don't have a locomotive at *either* end, if there was a chronic safety hazard it would have shown up long ago.
And by the way, when the MBTA *did* run to Concord, NH, in the early 1980s, that was with standard push-pull consists and the train pushed southbound.
