As I said before, these are two very distinct games.
Rails Across America lets you build a locomotive empire across the United States in a time period from 1830 to 2020. The historically accurate scenarios in it require you to construct an empire by paying attention the vast number of little things the world of trains has to offer, like labor unions and transportation choke points.
This hardcore strategy game is much broader than other train company sims like Railroad Tycoon. In it you must build tracks, stations, and cities, amassing fortune and fame along the way. You can also take the game online with up to 8 players playing simultaneously in a very real time, very fluid, and very cutthroat world.
Trainz Ultimate Collection, on the other hand, lends a more hands on approach to the actual train itself. This is a straightforward train sim at its heart. You will learn more than you ever wanted to know about trains and how they work from this piece of software. The missions you play are all goal based, meaning you have to transport some type of cargo to some place in a certain amount of time, and it's up to you to decide which routes and which locomotives you will use in order to get there.
Trainz also offers another great take on the whole genre. It not only allows you to drive trains yourself, but it lets you create the world in which you will drive them. From a grid screen you can lay down just about any type of geographic feature imaginable, including mountains, rivers, canyons, forests, and train tracks, and then take a joyride along them and see the sites you have created.