Matthew understood exactly what I mean. Thanks for adding the notes. What I am suggesting will make economic benefits even now. Think about a train using the open wagons carrying coal in one way and loading wood for the way back. This is not possible now, and won't be possible even if using the cargo holds system from ships. It would take an insane amount of micromanagement - send train to depot, replace all of cargo holds, and send it again. Also it will disrupt the flow of cargo - there will be no wood waiting on the station, unless there is at least one train with the appropriate hold on the way.
So for the players the incentive may be - use universal wagons with lower capacity and higher load times, but use them both ways; or use specialized wagons with quick unloading, higher capacity, but with less opportunities to fill on return trip.