I tried Airport CEO but never really got into it. Too much focus on running the business and designing the interior of the terminal building, and not enough detail/control on designing runways, taxiways, hangars, lighting, and performing various airport ops. It's pretty nice if you're interested in a business sim though.
I do feel the space is wide open in transit based games. Transport Fever, for example, leaves a lot to be desired while still being awesome too.