This is called a Superway. Unlike a subway, which consists of a train of cars travelling in below ground tunnels, the superway is an extensible track that originates as the launchpad of a behemoth flying machine using some sort of propulsion system, be it electric motors or magnetic levitation or repulsion.

The first behemoth flying machine takes off and its aerodynamic design generates lift which raises it and its "launch-track" off the ground. The Launch track continues to extend as it is towed along an ever growing hypotenuse.

The track is then kept aloft by somewhat smaller airplanes that are propelled along its length, at certain intervals, generating substantial lift. These would be best deployed over a large body of water, and have buoyant properties for easy retrieval.

Perfect for science fiction or a desperate last ditch attempt to flee this doomed rock of ours.