Working station-to-station cargo transport

It's now possible to move cargo into a train and back out of it again into another station. It's still very awkward in a number of ways - for example you can't "Unload all" or change increments, but the concept is there.

Next up is actually modelling these item moves as 'transactions' so they can generate income for the player (or possibly charge if the cargo has an up front cost).

I also need to decide exactly how stations get the cargo that they accept/create. I'm not keen on the Transport Tycoon approach of stations just having an "accepts" field, since it's quite sterile and impersonal. Levels are also authored by hand, so the concept of accepted catchment areas makes less sense (there aren't going to be procedurally positioned industries etc.). It's possible that items for outbound transport are more akin to 'quests' that have to go to specific stations - but I'm not sure yet.