If you're referring to the Walthers cars, they are from approx. the 1980 era and later. Earlier versions were smaller and somewhat different. Several other manufacturers produced similar designs.
Mike Lehman
Urbana, IL
The Great Northern Railway acquired 100 of the Walthers' style cars in 1968 or so. They were painted black with a unique style of lettering (for the GN). As they were non-revenue equipment, they were given X-series numbers, denoting MOW. Can't quite recall the series exactly, but I believe they were the X-7000 to X-7099.