Welcome rfxcasey!
now, on to your questions. well, nowadays, to download anything, you need what is called a FCT, or First Class Ticket. It's like $25/ year, and, well i dont wanna say guarantees, but it gives you access to better upload/ download speeds.
The next thing is, although you bought Trainz, no offense, but buying it from steam is the worst thing you can do.
I haven't experienced it, but from what I hear alot of people have problems with downloading content and with updates/ patches.
I'm not trying to discourage you, just letting you know that you may have problems sooner or later.
Lastly, you must realize that Trainz is a 90% completed game. The 10% is the content part. meaning, trainz comes with a huge amount of content. But lets say you download someone's route. you'll also need to download the Locomotives, sessions, rules, rolling stock, type of track, trees, etc. Trainz is also a 90% completed game, because all the content isn't "set in stone." meaning, you can edit like 99% of the content in trainz to your liking.
So, in the end, have fun, the forums are the best place to get advice/ solve problems, and when it seems trainz is being a big pain in the A55

, just stick with it and you will find the answer.