Feature request title:
Underground Transportation Between Terminals
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.
Images, references or additional content: