August 12th, 2015, 06:22
You would need to make a xml in the spawns folder that builds your train. The cars are selected using the "Pick Type" that is in the .xdp file of your passenger car. For an example see some of the train spawn xml files that are already there.
After you have your train xml file built you need to have it as a choice in the railtraffic.spawns file if you want it to show in a campaign or if you are using random spawns while flying missions.
If you want to make a mission specifically for your spawns you will have to make your own .spawns file calling your train xml file for use in the mission.