There were financial advantages to having equipment purchased/leased by SSW vs. SP, due to their relative debt loads. I think the last units specifically meant for SSW use were the eight GP40s in the mid-60s. A major reason the SSW remained separate (at least on paper) was that it was not 100% owned by SP (I think it was 96-98%); the other owners were finally bought out by UP after the merger.

It made sense to have SSW own the same types of units as the SP, since they roamed over the entire SP system. In an odd twist, when the SD45T-2s were rebuilt, some SSW ones came out lettered SP and some SP ones ended up lettered SSW. BTW, during the gray/red era, Cotton Belt locomotives carried small "SSW" lettering in the pilot face long before the large nose initials came along.