I play many of these types of games (Locomotion being my favorite) and I will admit that I rarely use air. I really like the Dirigibles that appear in the early years and maybe futuristic Airships, but the planes and helicopters don’t do much for me.
However, consider a future UPS (or FedEx or DHL) DLC (have them sponsor it?) that provides overnight express trains that meet up with UPS cargo planes, to bring containers of parcels to UPS distribution centers to be delivered by UPS trucks the next day. The challenge is to build the infrastructure in such a way that overnight delivery is guaranteed! Now I would use air.