I don't have TS12 myself, but in older versions of Trainz, there was a traffic ON/OFF toggle button in the Splines tab.
In T:ANE, they moved the toggle to the route's Environment Settings, Location tab.
In either case, after using the button you normally need to wait a while until vehicles start showing up, or do some minor edit to the road spline such as shift a spline vertex.
Also, make sure the Region asset is not missing any dependencies.