Currently, the loading/unloading time works per convoy, not per vehicle, by taking the slowest vehicle in the convoy. This works when all vehicles in the convoy are being loaded/unloaded, but works less well with mixed trains when, for example, passengers (which are the fastest to load/unload) are mixed with, say, coal (which takes a long time to load/unload, especially in earlier vehicles).
One day, it would also be good to be able to detach/attach vehicles during a trip so as to enable already loaded freight wagons to join a freight or mixed train more quickly than their loading time (as the ability to reconstitute in this way is an important part of what trains are all about: or at least was until the last 20 years or so), and in a manner more realistic than in Railroad Tycoon (where empty wagons materialised and dematerialised as if by magic), but that would be quite a lot of work, I think, especially to do it properly to enable things like locomotive changes and multiple unit dividing/combining.