Geeky, The F unit is an older Atlas/Kato model. Nearly identical in every way to the Stewart F3's. When I bought that unit, it was in a box of consignment items at a hobby store for $35, extra details, custom painted and all. Wheels were worn out, so I swapped the shell and truck side frames with another Stewart drive I happened to have. Works wonderful, now I need replacement wheels for the other unit now.