Hmmm, forgot about that one.
There are two versions of 'Trainz Driver'. There is the West Somerset Railway version, and the Auran released version. The Auran one is actually called Trainz Driver (In big black writing on a blue box), where as the West Somerset Railway version is in a completely different box with West Somerset Railway written across it IIRC.
Either way, unless you specifically want the route, then there is no real need to buy it. The West Somerset Railway version is the only way of getting the content that is in it, where as the Marias Pass version you can buy the Marias Pass directly from the creators.
Zec