The lack of aviation and some maritime options is a result of the pakset still being a work in progress - The Hood is currently making his way through the boats slowly, but we all do this as a hobby, so have limited free time available. The intention is certainly eventually to have a full range of aircraft and boats. Of course, we'd be more than grateful if anyone could join us in making additional stuff for the set; it's all open source.
As to the large number of road and rail vehicles, their distinction of purpose is more readily apparent in Experimental, where there are weight limits, comfort, loading time and catering to consider. In Experimental, one needs different sorts of vehicles for outer suburban, inner suburban, long-distance and light rural work, which is why there need to be more distinctions. May I ask - in what eras were you playing?