I'll bet you dollars to donuts this was a woman with those 'ethnic' several-inch-long fingernails/caps. They would be far too thin to trigger most practical technologies of sensitive transit-car door edge.
I had a very similar experience on the IRT 1 train years ago when a conductor closed the doors on my wrist with my briefcase still inside the car, at 116th, and took the train out of the station without noticing. I took the immediately following train to 96th, transferred to the downtown express, and caught the local at 42nd, where the foon had the colossal effrontery to ask me how I could possibly have caught up to his train. Good thing I'm not a litigious person...
I don't think there will be a particularly good fix for this; perhaps differential-pressure-sensing patches adjacent to the edges that could be banged on to open the door gap an inch or two momentarily?
SF muni has to take new car out of service
http://www.sfexaminer.com/munis-newest-train-taken-service-passengers-fingernail-gets-stuck-door/
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