Hi John, welcome to the Forums and hopefully, welcome to Trainz.
Getting in to Trainz takes I believe, a little bit of alternative thinking.
I'll try to explain some but there will be much more for you to learn.
When you buy the version that you decide on, you will get some routes included.
They are complete and will run out of the box.
As well as that you will be able to build your own routes using the assets that came with it. (the ones that are used in the included routes).
Trainz is different from having a model railroad... say in the shed down the back of the garden, because you are not limited by room to one route... you can have many, many, many... you only load one at any one time (usually... though I have been known to have two open so I could see an asset was used in someone else's route so that I could get the name to allow me to find it and use it in mine).
As has been stated there ARE over 40,000 assets on the Trainz servers (Called the DownLoad Station... DLS) and all of them are available for free download, once you have registered your purchase.
However, the download speeds are very slow.
To speed the downloads up you can purchase a "FCT" which will allow you to download assets much faster.
The FCTs have a limited time now, so it would make sense to get one and download as much stuff as possible in the time that you have it.
Once you have downloaded the assets, you can save them, so even if you have a hard drive failure (provided you have backed them up) you can restore them from your backup and won't have to download them again.
After you purchase T:ane or TMR17, you will be able to access the Content Manager (its a part of the program)It connectes through the internet to the Download station and you can then see and select the content that you want to download.
You will see that the assets are grouped in categories such as Routes (that are groups of many other smaller assets), Sessions, (That run in particular routes) Track, Ground textures, Plant textures, Locos, Rolling stock, Scenery (Including but not limited to Buildings, People, trees and just paraphernalia) all of that stuff goes in to building a route and and running a session in it. and even more!
TMr17 is more like a series of model train setups.
T:ane does have that but T:ane also can be used like a railroad simulator.
There is an update coming for T:ane (very soon I'm told) and when that comes we will have the option of purchasing the routes that are in TMR17 as a package.
That will make it that if one wants to his copy of Tane will basically include TMR17.
Personally I feel I have plenty of Model train routes available to me anyway so I don't think I'll get the package anyway... but I guess my final decision will be made when I see the price.
If you get T:ane there are a couple of "flavors" as well.
I have "Deluxe", and I'm told it has more routes built in than the "Standard" has.
Lots of decisions for you to make.
If you ask you will get lots of help.
There are some really good videos on you tube that will let you have a look at Trainz even before purchase.
If you google "Trainz" or "Tane" you will see some.
Also there are a few route builders that like to post their work there and they give a prospective purchaser a good look at what to expect.
This Gentleman is currently showing a series about how he builds his routes... This series is designed to show how he is building but in most of his videos he does go for a track run as well.
https://www.youtube.com/channel/UCuIt-Mra7IvgaqOJT0mj1TA
He has a Route in TMR17 and apparently he plans to release the routes he is showing being built on the DLS when he feels they are ready to be released.
This one
https://www.youtube.com/user/philskene/featured
does not show how he builds, but his videos, just show off his completed works... most of his works are available for free download on the DLS though he does have one that is included in TMR17.
This is still just a tiny amount of the stuff that you will discover if you decide to enter the world of Trainz.
It can be frustrating... but its also a lot of fun.
If I can be of any more assistance Id be happy to help, if I can.
All the best,
Paul