Regions
Did you select "Default" as the geographical region when you created the new layout? If so, cars won't be generated. I had the same problem. I wasn't able to change the region in the game, Trainz2009 (though I believe there is a way depending on if you have the most up to date version and service pack). I ended up creating a new route, specifying a region (in my case, U.S.) and then merging the existing layout into it. I made sure that the new and blank board was off to the side so I could just delete it, You can save it over the old name. Also, be sure you're picking roads that generate cars. Not all do. Maybe experiment with a couple different types. If nothing happens, it's probably the region. Good luck!