Here’s a good question: Can you use Lionel O or O27 transformers (or, for that matter, American Flyer S transformers) with HO or N scale trains?
The answer is, not directly. It will make a terrible noise if you hook it up. But you can make it work properly if you add a bridge rectifier. Look for one that’s 10 amps or more; don’t expect to have to pay more than a couple of dollars for one.
Connect the leads from the O/O27 transformer to the center leads of the rectifier, then connect the leads on the rectifier marked + and – to the track. If the train runs backward, change the leads.
The upside to doing this: O gauge transformers are available in higher wattages than you’ll generally find from the smaller scales. And Marx and American Flyer transformers are very inexpensive.
The downside is that you lose the ability to reverse the train, unless you add a DPDT switch to the circuit.
Flip the DPDT switch over to see the leads. Imagine the leads are numbered like so:
Connect pins 1 and 6 with a piece of wire. Connect pins 2 and 5 with another piece of wire. Use different colors so you don’t get confused. They’ll make an “X” across the leads.
Connect pin 1 to one rail of your track, and pin 2 to the other rail. Connect pins 3 and 4 to the + and – leads on the rectifier.
Once you have this wired up, you can use the switch to change the direction of your train.
Projects like this really make me miss Radio Shack, but there’s not much we can do about that now. When you’re buying parts like this off Ebay, sticking with US sellers will get you faster shipping, though far-east sellers usually offer the best price. This project shouldn’t cost more than $10 to complete.