I have a set of their scale length cars. The bodies are about 19 inches, the 20 inch measurement is from the end of the couplers, so it's not really scale length like the GGD or old Walthers cars are. They are a little over a half inch longer than the K-Line cars and about the same length as the Pullman cars Lionel ran about 10 years ago.
I don't know this for certain, but from pictures I've seen and the window arrangements it looks like MTH wound up with those Williams car molds and sold them with a matching diner. I've never owned the MTH cars, so I can't say for sure, but they look identical.