I don't think this would make sense from a logistic point of view. I don't know of any property Universal has right now that would generate an attraction as popular as Diagon Alley would be. Diagon Alley, when it eventually comes, will be a guaranteed crowd pleaser/generator, and it wouldn't make sense to have it on the upper lot right along side Hogsmead, the other main crowd pleaser/generator. They need to spread those crowds out, get people down to the lower lot. That's where they really need to build Diagon Alley.
And there's nowhere in any of these plans where the Hogwarts Express can be deemed unnecessary. I think it's very necessary. They're even including it in Beijing, where the two lands will be right next to each other.