Have fun with your trains
vsmith - if you are looking for a two-axle quality mechanism, there is only one source I know:
sb-modellbau
The page is in Germanm so you my have to run it through a translator.
Their drives are the best there is, but quite pricey, though.
The late model B-mann Plymouth switcher gets good reviews. If it's not part of your Train-Set starter stuff try to get one on the cheap. It's 6 wheel, but if it fails on the curves you can look to blanking the center wheels.
And it's a lot less than 140 EUR.
Come to think of it, the Minitrains 0-4-0 chassis is a pretty good performer. The Brits use it quite a lot for kitbashing (bodging) projects.
eBay store, Plaza Japan, has a number of good motorized chassis, and the price is right (even including shipping). Japanese N Scale (1:150) is quite compatible with U.S. N Scale (1:160).
"The trick" is to figure out "the best fit" of chassis to your engine shell. See some links here at 1950s Subways and Motive Power (http://cs.trains.com/mrr/f/13/p/202357/2212610.aspx#2212610).
Conemaugh Road & Traction circa 1956
Kato used to make two rather toy-like N scale locomotives for a toy train line they had. One was a four-wheel boxcab electric loco, and the other a six-wheel steam tank locomotive. (I do not know if Kato still has them.)
Some years back, I bought two of the boxcabs because they were cute I found them appealing, and I thought they had kitbash potential. They ran as well as Kato's more typical offerings.
I wonder if the Spectrum gas-electric might have some potential.
Dan
Also, if you can find one, the old Rivarossi 0-4-0 ran rather well. But those haven't been around for years.