But there was something about the compartment that seemed subtly different. I took a couple of turns around the small room, studying the bed, the lounge chair and swivel computer, the curve couch, and the half-bath as I tried to figure it out.

And then it hit me. The compartment smelled fresher. Fresher, cleaner, and somehow more sprightly.

I stepped to the display window and looked out. The tracks in the super-express Tube were arranged slightly differently from those in ordinary Tubes. There were only six main tracks, for one thing, with the Tube itself being correspondingly somewhat narrower. A set of auxiliary service tracks paralleled each of the main tracks about five meters to the right, which the official brochure said were for tenders and other emergency equipment. That made a certain amount of sense, given the thousands of light-years we were about to traverse without a single station along the way.

Still, I couldn’t help feeling an ominous undertone to the emergency-vehicle idea. I’d never heard of a Quadrail engine failing during a run, but just because it never had didn’t mean it never would, and with my luck it would probably happen on a train I happened to be aboard at the time. It would be bad enough riding for six weeks with a Modhran mind segment without throwing in an extra week sitting dead on the tracks waiting for a new engine to be brought up.

There was a subtle puff of displaced air, and I turned to see the wall separating my compartment from Bayta’s sliding open. The curve couches on either side folded into the wall as it collapsed, the whole thing depositing itself neatly into the narrow space between our half-baths.