As far as I know the only 3 rail conventional 1:48 proportion E8s were done by Weaver in the early 90's which became the basis for the MTH ones. Do you have a specific road name in mind? I have an A-A Amtrak set in Phase II paint I plan on selling sometime. I'm keeping the B unit and lowering for use with my 3rd Rail E8s.
I also believe that MTH offered the Railking E8s in conventional as well. That or you could find a Protosounds version and put a simple reversing board in it.
Good luck!