Unknown as to what the trigger is (I believe it is due to using multiple switches in a rail system with multiple trains), but Locomotives get detached from their freight cars but act as if they're still attached, sharing acceleration and speed of the parent locomotive even though said locomotive is in an entirely different location.
Video below demonstrating the issue:
If the locomotive has stopped at a station, the freight platforms do successfully unload / load the freight cars, even when they're not physically present at the station.
To me, this seems like just a visual bug or a logic bug with how freight cars are split down rails, not following their parent locomotives. Functionally, the trains still work, it's just visually broken / hilarious (ghost trains anyone?)