And a minute later, for the first time in over two years, I was standing inside the greatest engineering feat the universe had ever known.

The station’s general layout was prosaic enough, and aside from the fact that it was built into the inside of a huge cylinder, it would have felt right at home beside any Earth-bound train or monorail yard. There were thirty sets of four-railed tracks spaced evenly around the surface, with groups of elegantly designed buildings set between them that functioned as service centers, maintenance facilities, restaurants, and waiting rooms for passengers transferring between different lines.

Why four rails were needed per track was one more mystery in the Quadrail’s stack of unanswered questions. Two rails this size were required for physical stability, and a third could be explained if power was being run to the trains from an external source. But no one could figure out why the system needed a fourth.

Most people probably never even wondered about it. In fact, at this point in their journey, most people didn’t even know the tracks were there. The first thing everyone noticed when they first entered the Tube was the Coreline.

The official rundown on the Quadrail described the Coreline as an optically coruscating pipe inside the Quadrail Tube of unknown composition and purpose, which was rather like describing a bird of paradise as a flying thing with colors. Ten meters in diameter, glowing and sparkling and flashing with every color in the spectrum—including deep infrared and ultraviolet—the Coreline was like a light show on caffeine overdose. At apparently random intervals the pattern changes increased in speed and intensity, and most people swore they could see the thing writhing like an overtensioned wire getting ready to snap. The loose wire meshwork that encased the Coreline another dozen meters out added to the illusion, looking like a protective safety screen put there to protect passengers from shrapnel if and when the thing finally blew.