I'm using 2010 but I believe 2009 is the same. Open your route, top left of screen click on Main Menu. From list click Edit Route. Where it says Geographical Region use the arrows to see the different regions. When it shows the one you want, save it. This changes the region for the loaded map only.
When you start a new route it opens this same window. You can change the region where it lists the Geographcal Region.
Each new route you create starts with the default region and has to be changed to the region you want. I find no option anywhere to have all newly created routes start with something other than the default region. The only other possibility is if a line could be added to the trainzoptions file to start it in a different region. My guess is that it is NOT an option. But, as I have very little experience modifying the trainzoptions file, perhaps someone more experience could comment on this.
Doug