“So,” Callum said, “the train stops are malfunctioning. The train stops are the controls at the beginning and middle of the track at every Tube station. If a train approaches at anything faster than ten miles an hour, the switch is tripped, and the train stops automatically. Now, this is really important. Look down. How many rails do you see?”

I looked down. I saw three rails—two of track, and a third, heavier one running through the middle. They were all resting on blocks of some kind, about two feet off the ground.

“Three,” I said.

“Okay. Best bet, don’t step on any of them. But the one you really can’t step on is that third one, because you’ll fry. The trick is you walk in the space between the rails. It’s wider on this side. Walk really, really carefully. It’s not complicated, but if you mess up, you’ll die, so pay attention. You wanted to learn. This is how you learn.”

Callum smiled slyly. I wasn’t sure if he was joking. I decided not to ask. I followed him down the steps. The entrance to the Tube tunnel was in front of us—a semicircle of light black that led into an unknown pitch-black. Callum put a flashlight into my hand.

“Keep it pointed forward and down. Walk slow and steady and don’t jump if you see a rat. They’ll run from you, don’t worry.”

I did as he said, trying to act totally unconcerned about the electric rail or the rats or the dark. Once in the tunnel, the temperature immediately dropped a few degrees. About twenty feet in, there was a man. He was right between the rail and the sloping brick wall of the tunnel. He wore a rough work shirt and boots, loose gray flannel pants, no coat.

“I hate this station,” Callum said under his breath.

When I shined my light directly onto the man, he was harder to see. He was so pale and fragile, he was like a trick of the light, a kind of visible sadness in the dark of the tunnel.

“Listen, mate,” Callum said. “I’m really sorry. But you’re going to have to stop messing with that switch. Just stay away from it, all right?”

“My family . . . ,” the man said.

“A lot of times,” Callum said, never taking his eyes off the man, “they don’t even mean to do the things they do. Their presence just interferes with the electronics. I doubt he even knows he’s been tripping the switch. You didn’t even mean to do that, did you?”

“My family . . .”

“Poor bastard,” Callum said. “All right, Ror. Come closer. Up here.”