Which it did, accompanied by squealing so much louder than the usual that I stuck my fingers in my ears. (At the time, I thought the driver must be taking the bend too fast, but I can't be sure that I had any visual evidence for that, or just jumped to that conclusion as an explanation for the extraordinary squealing.) As the back of the train came into view, I saw, roiling along behind it, a cloud of black smoke. I saw no signs of flame, nor of smoke coming from the train, so I decided the smoke was consistent with my took the bend too fast theory, and turned to walk up the platform a bit as the train slowed to a stop.
By the time the train had stopped and people were getting off, the smoke had billowed in at least two car lengths, still pretty thick and reminiscent, more than anything else I've ever smelled, of a tire fire. But I still wanted to get home, and one thing I could see clearly was that the air in the car was a lot clearer than the air on the platform, so once people finished leaving, I got on. Only to be followed 30 sec later by a T employee telling us to get off.
By now the platform was a sea of people. all sort of vaguely headed down the platform — which, being also the direction the smoke was coming from, probably explains why they were moving so slowly. Luckily for me I lived in Harvard Square for several years; I headed the opposite direction and took the escalator to the stairs to Holyoke Gate, and was in the fresh spring air far sooner, I'm sure, than any of the people who'd been drifting with the crowd.
I ended up taking a bus to Lechmere and another bus home; probably got home about a half-hour ago. So it's now been nearly two hours, and none of the google searches I can think of produce any results under "News". So aside from a few hundred offended noses and delayed commutes, probably not a lot of harm done.