Design of the Day: Airborne Metro - Flying Airport Concept

Airbus A-380 showed that with the increase of a vehicle's size engineers are also increasing its efficiency, making it eco-friendlier. Following this logic in the near future huge aircrafts will be much more efficient.
One of the most daring projects in this field is entitled "Airborne Metro" and it involves the use of a large airship that will play the role of an airport. The craft is so big that other smaller planes will be able to land on it.
The designers of "Airborne Metro" concept say that the aircraft is like an express train in the sky. But apart from the train system we know, the Airborne Metro does not take passengers directly to a certain destination, but serves as a place where smaller vehicles with passengers land and where people can take another plane to get to their final destination.
The huge flying airport is able to carry up to 3,000 passengers. In order to transfer passengers to a plane, the latter would either have to land on the Airborne Metro or make use of a special docking system.
Undoubtedly such a huge aircraft would require a lot of energy to take off and land. In order to make it more efficient, the flying airport would most of the time stay in the sky, being powered by a nuclear reactor of some sort.
It has been estimated that the project lead to an overall fuel saving of up to 40 percent for a 600-mile journey and up to 80 percent for a 6,000-mile trip.