Nothing too fancy a suggestion here, since it's in Advance Wars. But it'd be helpful if there was a sound effect and an icon/indicator ont the unit itself when a wagon/barge/balloon gains a passenger. In AW it's accompanied by the sound of a car door closing, maybe the wagon could creak or something. I think in AW when a transport has a passenger it has a "lock" icon of some sort on the unit. It's totally my fault, but sometimes I do come into situations where I park a wagon outside a barracks for an infantry and then on my next turn I speed it away without its passenger because I assume I'd already loaded it.
