The only thing I can think of is that the PS-1 was a very common car of the 1950s, but that doesn't mean the paint schemes are 50s. I think most modelers, not familiar with a particular road, would assume the dates are for the paint.
I've gotten to the point that, if I'm not familiar with the car, or the road, I look for pictures online before ordering the car. If I can find evidence that the car was probably in service in 1974, I'll consider buying it.
I'd buy an X53, in PRR or PC, just to have more variety in my interchange traffic. Obviously the PRR versions were still in service in 1974.