Hi Paul. The answer is, probably whatever was the cheapest and readily available. Probably a Red Oxide color would be best, but refer to Rule #1. "It's your railroad". Unlikely you'll find anyone who actually saw it.
Ha! good one Mike.... however Yes, there are colour images taken there "out there"; most likely by Muriel Wolle, or Robert L. Brown; and no I don't remember just where (yet), and don't have time to chase this goose today.
I have taken the easy option I used artist red oxide and dry brushed the walls so the wood stain can be seen underneath.
will insert a photo later. Paul R.