When we put engine on some mission but have not enough money for waggons, it goes not with full capacity.
Later when we try to buy waggons there is only option to buy all of the rest. Green button to buy is inactive if we dont have funds for all remaining waggons.
My proposition is to add a button to buy as much of waggons is possible for a moment, for example 2 of remaining 5.
So one button can be as it is, inactive to buy all remaining ones. And another new would be to buy as much as possible for the moment.