in many big airports in the real world, passengers can use a train or metro system to go from terminal to terminal. i think this would be very cool as right now there are no ways for passengers and staff to get from building to building without actually walking out of one and into another.
there should be a train/metro station structure as well as above-ground and below-ground train/metro tracks. there should also be different types of train you can order from procurement, that can be stored either in a train/metro depot or a large vehicle depot that would combine the functionality of a regular vehicle depot and a train/metro depot.
it would also be cool if there was a train/metro track coming in to the map at the start of the game, similar to the starting road. you could connect a station in your airport to it, to have passengers arrive by train/metro directly from the city.
if you have any ideas to expand this idea or any criticisms of what i already have here, post them in the comments below! thanks for reading!