doberman, my advice is to always do a little reseach on line or with other train guys before you buy something, if you have any concerns or questions on a product.
Even Tom's point about MTH is a little flawed. While the 20- and 30- numbers do accurately reflect the Premiere and Railking product lines, they do not accurately reflect the scale proportion of the product. Railking was always semi-scale - typically larger than a Lionel semi-scale car, but still semi-scale.
Now that MTH has been using some of the older Premiere line dies for Railking products, many are scale sized, though still have the 30- number. If you really like the Railking scale and then unknowinly order an older Railking locomotive with a 30- number, you could be disappointed.
To further complicate things, MTH has the 33- Rugged Rails line. The Rugged Rails rolling stock are right on par size wise with the more traditionally sized Lionel products. The engines are from the smaller older Railking dies. Though they have not cataloged any separate sale Rugged Rails products in years, they still put them into train sets. So you can get a Railking Scale train set that could still have a much smaller Rugged Rails rolling stock car in the set, even if the set has a 30- number.
It's not just MTH. Lionel does this too. The recent CP Rail starter set has scale sized cars with an older MPC era tooling based locomotive. Or the BNSF set, which had larger semi-scale cars, but then came with a most definitely smaller SP-type caboose.
So regardless of a company's product numbering system, it still pays to to ask questions and compare photos of a item you are interested in.
KD's advice is well taken too. When I had a local shop that carried MTH, he had a display layout with all his different lines represented. When I'd see something I liked, I'd ask if we could put it on his layout so I could see the size of it versus other semi-scale cars. That meant sometimes I didn't buy. But it also meant, when I did buy, I was going to be a happy customer and give that dealer a good word to my friends.