Donald --
Sounds like a very cool trip, and I envy you! I think you could make the 21 work for a lot of shots, both inside the train and in the train looking out. As I was riding home from SF on Amtrak on Saturday, I shot a view frames and thought about how to capture station scenes, buildings, and other structures from the vantage point of a passenger train car (and a moving one, at that). I think if I were to attempt your project, I'd include a normal or slightly wide-normal lens in addition to a genuine wide angle; both for tighter framing and also for a more neutral perspective (I happen to love 50s for landscape photography). So I'd probably throw a 40 or a 50 in the bag, along w/ the wide angle. But that might take away from the challenge of doing this project w/ just one lens, and if you know you like shooting the 21, I'd certainly go w/ that.