I do not know if it is still available but the Train Engineer unit appeared to be the simplest unit available to convert and control a conventional engine remotely. It was originally developed by the now defunct Aristocraft trains but is now being marketed by a new company called Crest Electronics. I have not seen much advertising on the system of late but you can see it here: http://www.crest-electronics.net/home.html
Downside of the TE system was that it did not have a conventional fallback. With the TE board installed, the engine can only be run with the TE remote.
I spoke with the Aristo folks about this at York once. They didn't think it was a drawback--just carry the remote and run the engine anywhere there's powered track.
---PCJ