Actually, they make 3.
Proto is their top of the line brand, highly detailed and very smooth running.
Mainline is the mid-range. It uses the same mechanism as the Proto, so they are very good runners, but there is less detail on the models.
Trainline is their low end brand. Most of the detail is molded on, and it uses a cheaper drive train, which isn't as smooth. Still a good value, though, I'd you can get past the lack of details.