THEN ONE DAY, AS THE TRAIN WAS PUFFING ACROSS A plain of scrub so flat that it looked like somebody had made it with a ruler, and the little pangolin had graduated to eating bits of raw hamburger from the dining car—which they were all very proud of him for—Kate strolled back through the passenger cars and noticed they were looking emptier than usual. Fewer animals were getting on, and more were getting off.

A few days after that they were down to just the animals from the library: the fishing cat, the white-bellied heron, the green mamba, and the porcupine.

Plus the sleeping polar bear. And the baby pangolin. That was when they started what was in some ways the hardest part of the whole trip.

They crossed into a frozen desert, mile after mile of empty sand dunes covered in thin tiger stripes of frost and snow. They steamed across it for days. Dry powdery snow and sand whispered and rattled against the windowpanes when the wind blew. The porcupine was grumpier than usual and complained that he wasn’t getting his fair share of time with the pangolin. Once they came to a tunnel with a sign outside that said DANGER! FALLING ROCKS! and Kate and Tom had to get a squeaky old handcar out of one of the boxcars and pump it along ahead of the train all the way through the tunnel, in the freezing cold, to make sure the tracks were clear.

Another time the Silver Arrow almost ran out of water, till Tom remembered that they had a whole swimming pool car full of it.

At night either Kate or Tom would sit up with the Silver Arrow while the other one slept curled up in their cozy bunk in the sleeper car. They kept an eye out for dangerous curves and warning signals and steep slopes and anything that might be blocking the tracks. More than once they had to stop and shovel away drifts of windblown sand.

It went on for so long that Kate started to wonder how much more she could take. There were dark shadows under her eyes, and when she closed them, all she saw was more track scrolling past her. She was so tired she kept bumping into hot brass pipes in the cab and burning herself. At the same time the bitter chill of the desert spread deeper into the train, so they could see their breath inside, and even when Kate huddled right up next to the firebox she still couldn’t seem to get warm.

Where am I? Kate thought. What am I doing out here? It felt like she’d been on the Silver Arrow forever. It was the adventure of a lifetime, but it was a whole lot of work, too. And it was taking a really long time.