The original Proto-1 versions like 20-3013-1 suffered from tragically tall, toy-train gearing. IMO the Proto-1 version isn't a good runner. Speed control is a must, and it will be working full time to prevent this long-wheelbase, rubber-tired loco from slowing unrealistically as it enters the O54 curves.
At some point after the original 1995 issue, I'm pretty sure that MTH redesigned the chassis, with a larger worm wheel as used on the UP FEF and NYC Niagara. These mechanical improvements make it worth holding out for one of the newer PS2 or PS3 versions. Problem is, they never advertised or publicized these improvements. So unless you want to download and swap a bunch of sound files, it might be tough to pinpoint which was the first version to have the improved gearing.
The 2900 is a huge loco. I would strongly consider a scale 3751 instead, if you don't already have one. For either of these, the 3rd Rail version would be my first choice, and Lionel did a pretty good job with their 3751 too.
