Excuse the following Potterhead-fueled rant.
When bringing a fictional land to fruition in a theme park, there are obviously going to be some adjustments made to stay realistic. For example, Hogwarts Castle is not located directly in front of Hogsmeade Village as depicted in Japan and Florida. In the books and films, on the other hand, it is located in the heart of a mountainous area of Scotland surrounded by a massive lake. Minor things like the geographic obstacles I just mentioned can be eliminated because they don't affect the story drastically. Not including the Hogwarts Express with Diagon Alley, however, would be. With Hogsmeade located in Scotland and Diagon Alley located in London, it would make zero sense to be able to travel from one location to the other in mere seconds at USH when the real destinations are situated on opposite ends of the United Kingdom. The Hogwarts Express not only acts as a buffer between the two, but an insanely immersive form of transportation; enough to list it as another attraction.
To put it briefly, introducing one without the other would be a big mistake on UC's part.