Trainz Driver is a version of Trainz without the route building capability, that is released on smart phones.
Trainz Simulator 2012 is the latest version of Trainz released on the PC. I also think that there used to be a gutted version of TRS06 called trainz driver several years ago, but I could be wrong.
Pretty much, if you want Trainz on you phone, get Trainz Driver. If you want it on your PC, get Trainz Simulator 2012.