Have you tried this with portal A returning trains via portal B and having portal B set to produce loaded trains only ?
If that does not work then it should be possible to do this with intermediate portals, something like:
Enter Portal A Loaded > Leave Portal B (off stage) empty > straight into portal C (off stage) as empty > return from portal D (on stage) loaded.
Or you could experiment with the "Instant Load" rule - InstantLoad_12,<kuid2:45324:555017:4> built into TS12.