That's a blockade done against human drivers, who (usually) know how to drive off the railway track, and the blockaders are only protesting rather than actively trying to murder. They stop cars from passing but don't trap them on the tracks.
What GP suggests is that by people simply standing there, the self-driving car's software will stop on the track without aggressively trying to escape.
Here in Poland we have campaign teaching people how to get out of a railway crossing if you get stuck. A bunch of differently-smart humans didn't even contemplate driving through the bar gate, and in some cases didn't even evacuate the car either. The bars are designated to break easily when forced by a car, but somehow in a stressful situation drivers regard them as sacrosanct. As Waymo cars behave that way in about every potentially dangerous situation, I'm afraid they'll do the same when on a railroad crossing as well.