In real airports like London Heathrow and Atlanta Hartsfield-Jackson there is underground transportation between terminals. In the game those could be as simple as an underground sidewalk or as complex as a whole train system with a depot and necessary fuel. It would allow terminals that are not connected to the main terminal but let them still be functional.
Why it should be implemented:
To make use of more space in the area and to make larger and more realistic airports.
This could be something implemented after the multi floors update, and it would be incredibly useful to get passengers to far flung areas of the airport. That being said, I don’t think this would be very easy to code, so we may not see it for a while.
PIT (Pittsburgh International Airport) has four trains that transfer you from the ticketing area and baggage claim to the hub where all of the gates are.