In recent news, the MBTA is going to pursue a policy of front door-only boarding and alighting at Green Line surface stations, as a means to stop people from dodging the fare-box. I find that people get upset and irrational about fare evasion. I wouldn't be surprised if many of the people that seem to be "evading" are actually monthly pass holders looking to board more conveniently. It's unfortunate that nobody seems to be able to put a solid number on the losses. There is no cost/benefit analysis to this campaign. It may very well cost more to slow down operations by forcing everyone through the front door bottleneck.
Still, it may be worth a shot to see just how bad it gets. Although not formally recognized, peak-level congestion for the "B" extends well into the late evening, due to the large numbers of students riding. It is common for outbound trains at 9pm to be at crush loads during the semester. But, with careful discretion, it may be possible to implement this policy while opening all doors where necessary. Already, for the past year, drivers typically delay opening the rear doors by 5-10 seconds. It seems to produce a psychological effect which motivates people outside to move to the front door, while creating a slight amount of consternation for those waiting to alight. On the other hand, with the semester already over, they may not get to really test it under load until September.
By comparison, the fare structure in Pittsburgh is oriented around front door-only boarding and alighting outside of downtown. The idea there is to allow opening all the doors where the bus may find itself most congested, and balance that by closing the rear doors where it should be less congested. This is somewhat similar in rationale to the old system in Boston where outbound street-level trolleys did not collect fares. This new initiative by the MBTA doesn't pursue the goal of alleviating congestion, however. It seems to be an overreaction to the perceived problem of fare evasion.