There's two types of 40' stock cars you can get that I know of; one type is slightly shorter, has fewer planks, and doesn't have a wider plank about halfway up the car, like this one:
The other kind is slightly taller, usually has narrower planks, has a slightly wider plank about halfway up, and has more planks in general, like this one:
These nests fit the shorter ones perfectly, but the taller ones will not fit in them. I'd recommend just getting a troop car nest for any tall stock cars.
And if you aren't sure whether your stock car is 'short' or 'tall', just keep in mind that these things are super inexpensive compared to shipping costs..