They don't sell it.
We are in the year 2021, 15 years have passed since Trainz-2006, which in computer science is equivalent to a century. For Trainz-2006 to work fairly well you will need Windows XP, Vista and Windows-7 forcing a bit.
I tried to install it on Windows 10 once and it didn't work out.
Trainz 2006 may be found in a second-hand stores, street markets and perhaps used online and will surely be already registered.