Many airports have features that are below the surface but are still visible from above ground, such as tunnel entrances, etc. It would be nice for them to be included in the app. The specific feature that prompted me to make this post is the train in terminal 3 at ZBAA, which I have actually ridden on before IRL.
I can understand if this will take time to implement, because it requires overriding the terrain, but airports already do that over uneven terrain, so maybe it is straightforward.
3D airport/object requests are currently not allowed (including airports, ATC facilities, buildings and objects etc.)
Due to this, this feature request has been closed and all votes returned to their users so that they can be used to vote on other features they would like to see in-app. For more information on what is permitted, please visit the About the Features Category.