jawyatt:
When reading the whole paragraph, it is a little clearer that the text was referring to the SP Heavyweight Cars listed on the same page of the 2007 catalog.
“Decorated for service on the Milwaukee Road AND Southern Pacific mainlines, this signature varnish is prepared for service with the famous Bi-Polar AND GS-4 steam locomotive”.
Since the GS-4, Bi-Polar and the respective heavyweights, were all listed on the same page, it is pretty clear that the wording is referring to the SP cars for the SP engine (GS-4) and the Milwaukee Road cars for the Bi-Polar.
Without pictures, I can see where the wording may be a bit confusing.
Regardless, if you are looking for cars to pull with a GS-4, look elsewhere. If you are looking for the proper engine to pull with these Milwaukee Road cars, you can choose from the Bi-Polar or MR 261 Northern. There may be more, but that is a start.
Hope that helps in your decision making.
Charlie