Well, I've never tried it, but it seems like it should be possible. Some things that come to mind that you would have to address:
1) the motor does not need to be switched out, but you would need to install a rectifier to covert your AC track power to DC for the motor. You will also need some sort of reverse unit, then. In DC, you simply switch polarity to run the motor in reverse; with AC it's more complicated. You could solve both problems at once by installing a command/legacy upgrade kit.
2) The wheels on the left side of a two-rail loco are insulated from those on the right. This means you will be grounded only on one side, which could cause problems if you are using insulated rails to trigger signals, etc.
3) 2-rail locos have itty-bitty scale-sized wheel flanges. You may be able to get by if you use track with a realistic T profile and high-quality turnouts. But even then you could run into trouble. IIRC, many scale locos have flanges on all wheels--i.e., no "blind" center wheels. This would seriously restrict the radii you can use (e.g., the original Lionel Hudson has a minimum radius of 36", or O-72). You may have to replace the wheelsets altogether.
4) And of course, you will need to install a 3rd rail pickup!
Given sufficient time, money and patience, it's probably theoretically possible. But would it be worth the amount of labor involved? Not to me.
That's pretty much what I was saying, but in fewer words
If you use track where all three rails are isolated from each other (like Atlas track), you could set the track up to run either three rail or two rail. The problem would be if you were to forget to switch to three rail operation when putting a three rail engine on track powered for two rail, or three rail rolling stock on track setup for two rail.
The motor may already be suitable for AC. Universal motors were used for both AC and DC long before permanent-magnet motors became practical. If the motor has more than two terminals and you can see that it has a laminated magnetic circuit, it is probably a universal motor. If not, a bridge rectifier will do the job: http://www.radioshack.com/product/index.jsp?productId=2062580
You can use a traditional Lionel E-unit or an American Flyer remote-control unit for reversing either motor arrangement. (They are functionally the same.) The Flyer unit is smaller and easy to mount horizontally, which helps when the locomotive was not designed for the vertical Lionel unit.
There is also a harder-to-find horizontal Lionel unit. I use one in my Rail King Big Boy, which has surprisingly little headroom.
The wheels are indeed insulated side to side. But they don't have to stay that way. You can easily connect them together.
The one 2-rail DC (ETS) locomotive that I have converted has slightly smaller flanges (but not "itty-bitty") and a series-wound motor, although not universal; so it runs on DC rectified from the track voltage and reverses with a Flyer unit.
Bob Nelson
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