I've seen that in other places. It was probably more common in this era just because there were more industry tracks with very sharp curves.
It stands to reason that the back of the wheel flange would contact this rail, if the truck were to start to fail to follow the curve. This is probably a more effective restraint than the opposite wheel on the opposite rail, which is of course also a restraining lateral force in the same direction as this guardrail is exerting, while the truck is rotating with respect to the body of the car.
