Imagine that there are only two mega cities left in the world: Sanctus Domus and Spiritus Lenis-- two enormous underground cities. The Earth's surface is left desolate and uninhabitable by a nuclear fallout caused by the 3rd world war.
Connecting the two cities is a long, underground railway. Our hero (or you, if you prefer) is taking the train to Sanctus Domus to investigate recent rumours of a megacorporation's corrupt activities. He sits staring out the window as the train zooms through the tunnel.
Then the tunnel turns upward, bringing the cabin to the surface for just a few minutes and he gazes across the desolate, sandy wastelands and upon the ruins of what had once been a city of mankind. It makes him think of just how much damage mankind had wrought their home and imagine what it used to look like (for he had been born and raised underground and never seen what the Earth once looked like).
A thud from below shook him awake as the train dove into a new tunnel.