I’m looking at getting this, but am confused by all the versions out there. Is this version an add-on, or complete by itself ?
http://i14.photobucket.com/albums/a317/JPancoast/TRAINZBOX.jpg
The current version is Trainz 2006. The you have linked is from several years ago, there have since been a few upgrades. Trainz 2006 is a fun program, lots of customization and the route creation tool is very flexible. I’ve been creating a virtual version of the route my layout would take for a while, almost done! 
the one you want is here
http://www.auran.com/TRS2006/
I have the earlier one called ‘ultimate trainz’ and I like it.
Make sure you have a powerful machine though. Otherwise you will be disappointed. I have a 1.13 Ghz CPU and it barely does the older one.
Just a heads up.
It is good though.
Thanks guys !