The rectifier doesn't have to be IN the train. You can do that behind the present ac trasformer on a toggle for ac/dc and second on/off/on 3pos or dpdt toggle for direction changing.
I think I am going to toggle my DC power supply to the track. I have been testing with a P.H. Hobbies PS3 filtered DC 3 amp power supply that I have for G gauge trains. It runs the engine very smoothly and has fine slow speed control. I was worried that it had too much power for this engine since the engine is rated at 12 volts and the power supply can put out 20 volts, but the throttle seems to have nice voltage control at the low end of the range.
One thing I will need to do is to ensure that the section of track I use this on is completely isolated from a separately powered block that could be running AC with a different train. I usually connect loops with switching track and it could cross over a Standard gauge loop.