Many people use the instant load command (InstantLoad 09,<kuid2:45324:555017:1>, InstantLoad,<kuid2:67585:1017:1>) to load AI trains from portals but the load will be the first one mentioned in the config.txt files load container.
Normally trains spawned from a portal use the default load settings for any vehicle which usually is empty. You could make a clone of any vehicles you want to spawn from portals and edit the config.txt file to have any load you want and its quantity, just make sure it is the first in the list if you have more than one load if it is to be used with instant load.