1. Got to the website trainzportal.com
2. Add to cart.
3. Create an account if you don't have one.
4. Checkout and wait for an email.
Email takes 15 minutes to arrive. After this the link my account on the website will update with a download link. The email contains your serial number.
From the website:
Limit one per person.
The first release of Trainz includes Surveyor, Driver and My Collection