If you choose Default Region when building the route, you will have no cars. To fix it, try opening the route in Surveyor, on the menu in the upper left, there is a choice to change route properties (I think that is the name) try changing the region there. Or Open for Edit the route in CM2, open the config file and change the 2 letter code for catagory-region.
I'm sorry, but there is no such option in the Main Menu on the leftmost corner in Surveyor (Trainz 2009 V3 b.38411). You can only select a region when you create a new route.
I have managed to Open-for-Edit the route file, found the config file, and saw a line specifying 'region'. There is no variable in the righthand column, so I need to change that. So far, so good. But: change into what? Presumably a (two-?) letter code for a region.
Any suggestions would be welcome!