Jimbob, Check out the thread on 130' Walthers TT. It has info on the 90' as well. These units are ready to run.
Sue
Anything is possible if you do not know what you are talking about.
If you are patient and enjoy challenges, then get one of the kits and motor kits. I got mine to work, but it was like a Masters program in mechanical engineering....well, okay, you know what I mean.
My advice- if you won't choke on the wad that it will take to get one into your layout, spring for the built-up digital model. Select location, measure hole, cut hole, place TT, make connections to power, read manual, and calibrate the bridge encoders. Then, toot the whistle of the loco that makes the first traverse of the bridge.