"Has Anybody Converted a Brass 2 Rail Steamer to 3 Rail?"
I've seen folks go the other way from 3 rail to 2 rail where the problem becomes turning and insulating the drivers. In your case I think the main problem might be finding room for 3 rail size flanges, many steam locos have very little clearance between the flanges of adjacent drivers and other parts. Perhaps you could add 3 rail wheels to the leading and trailing trucks and see if the 2 rail drivers stay on the track? If I had a 2 rail brass loco and wanted to run it on 3 rail track I'd probably sell it and buy a new 3rd rail loco instead. Another option is many of the early O scale locos were sold as outside 3rd rail which would just need some center rail rollers added to work. ....DaveB