Trainz/Changing the default cars on roads
< Trainz
You may need to change the region but you can add specific cars or trucks if you know their kuid number by modifying the layout or route's config.txt.
An example is:
region <kuid:-1:7803>
carrate 20
car0 <kuid:329364:1501>
car1 <kuid:329364:1165>