"Yes, the clouds of steam that gather every time we stop at a Void platform suggest it's a steam engine, but I don't believe it for a moment -- unless it's being generated by matter-antimater fusion or some other futuristic engine type. Otherwise, the train looks as though it was built in the 1800s. But I think that's just the artistic taste or historical interest of the builder."

He stares as the Doctor licks the metal wall.

"What the heck are you doing???" he asks, with a grin.