For custom traffic in 2004, go into Trainz Objectz programme, wait paitently for the kuid check, then go into "Maps".
The programme will automatically list all routes available to you, including the 'standard' ones supplied, any you have d/led from the DLS, and, finally, any routes that you yourself might have created.
Right click the map you intend to customise with traffic, and you will see a menu appear. Look for "Add/Remove."
Click on this heading, and the programme automatically re-arranges all your Trainz content. Scroll down the list until you find the vehicle/vehicles you wish to use, and highlight a vehicle. Then, press 'F7' key and the vehicle will be added to the list of traffic on that route.
Be careful, though, because some cars/trucks do not run in the correct way, (i.e. Forwards). A few will run sideways, and two that I have found to date that run in reverse. These are, 'Christine" and the Chevvy 1950 Pick up truck.
Luckily, most of the vehicles work in the correct way, and by using the above method, I have successfully "customised" all my main routes.