Sourdough's suggestion will increase the diversity of car types, but not the volume of traffic. For that you need to go to the roads config file and change the carrate number. For example, Road USA, -1 100598, has a carrate number of 35, ie a new car is generated every 35 seconds. You can use any number greater than 3, so setting the carrate at 4 would give you the greatest volume of traffic Trainz allows, a new car every 4 seconds. You can also set it to 0 for no cars.