For those who haven't been in the hobby for 40+ years, the "PC steam loco" started with Walthers, which once made decals in N, HO, and O for such a thing, along with BN and GN Big Sky Blue style. I think they had a 4th one, but can't remember what it was, if it existed.
I also seem to remember that one of the ex-GN/NP tie plants was still using a fireless steamer for switching, and if so, the BN would have owned a steam loco for at least a short time. I won't swear to that, though.
A Conrail steam loco, in Brunswick Green, probably wouldn't have been a bad looking locomotive. Not "traditional", certainly, but for the latter half of the 20th century it wouldn't have looked out of place. The GN BSB lettering would have looked fine, as the main change was the modern herald.