Your best, cheap bet is any of the "Express" train sets made by Goldlok. There were a nunmber of them in this series, including:
Western Express
Passenger Express
Patriot Express
Even a Christmas Express (I'd stay away from that one).
They are usually sets with a locomotive, a tender, and four more cars. There are subtle differences in the locomotive, and usually one car different (patriot express has a flatcar, passenger express has two passenger cars but no boxcar, etc).
Sometimes you only get 4 cars (loco, tender, two other cars), but extra items like more track (including switches) and perhaps station houses. The best ones are the 6 car sets.
Prices range from Under $10 USD to maybe $35 USD or so. I bought a Western and a Passenger set new for $15 USD each, plus postage.
Here's a Western Express on eBay for $8 USD (current bid) plus postage.
auction
They are "about" the right size, come with track, can be repainted if you want, and are suitable for gaming. They are not the perfect size, but close to it. And cheap.
They come and go, so if you can be patient, just do a search once a week, and eventually you will score one cheaply.
