Hi,
I would suggest that you wait for TS 2019 to be finished, before you buy it.
Many of us (as you can see from the header) have had many versions of Trainz, and are able to accomodate the possible changes/minor bugs that we find.
For a learning experience, I'd suggest T:ANE - or if you want just a taste, try the free download of TS 2009 (I think it's available for MAC)
http://ts2009.trainzportal.com/free/
Remember that it's about 10 years old, but it's quite good enough to get a taste of the program and its possibilities.
There is masses of free content on the "DLS" 0r "Download Station" - to download these, you will find that you need to register, and buy a "FCT" (First Class ticket) which gives you access to fast downloading (without a cap).
Once you have registered, use the same registration name for your Trainz life....
Hope you enjoy,
Colin