There is no unload all, and there won’t be, there is no reason. Goods and people automatically transfer between stops to get to their destination; forcing a vehicle to unload would have no purpose. Also, there are no disadvantages for a vehicle having cargo both ways, weight isn’t actually counted (yet) and the cargo automatically move by themselves (you can see where they are if you turn on the cargo layer) and your carriage must be faster to get to a town near the forest than if the goods moved by themselves.
Train Fever
Official Website
