They knew these woods like the backs of their hands. They’d lived here their whole lives, and they’d climbed and jumped off and fallen from every tree and rock a million times. But they’d never seen the woods at night from the cab of a giant runaway steam engine. Kate braced herself for the ultimate smash, when they would hit something big or when the tracks would run out and the whole train would lurch to a halt. It was going to be such a disaster. But so worth it. She swore to remember this her whole life: the night she rode through the woods behind her house in her own real steam train.

But the big smash or lurch never came. Instead the train kept going. Birds startled. Stiff branches scraped against the windows. She and Tom laughed hysterically. How far was it going to go?

Then Tom stopped laughing.

“Wait,” he said. “What happens when we get to the hill?”

It was a good question.

In the old days, when people made maps and they came to the part where they didn’t know what was there, they just drew a bunch of dragons and sea monsters instead of land. On the very oldest maps they wrote Hic sunt leones, which is Latin for “Here be lions.”

About a quarter mile into the woods behind Kate and Tom’s house there was a sudden steep hill, almost a cliff, with a chain-link fence at the top, and at the bottom was a scary dark swamp with a lot of bugs and, supposedly, a giant snapping turtle so big it could bite your foot off. If an olden-days person were making a map of the woods behind Kate and Tom’s house, that hill would’ve been where they started drawing sea monsters. Or lions.

Kate risked sticking her head out the window.

“Oh my God. We’re almost there!”

“Kate,” Tom said seriously, “what’s actually going to happen, though? I mean really? Should we jump out?!”

“I don’t know!”

She felt paralyzed. Panicked. She was the older one, she was supposed to know! For the first time it occurred to her that maybe this wasn’t going to come out okay. She wondered how deep the swamp was. If the train hit that water and sank, they’d be trapped. They could drown.

But it was too late, because even as she thought it she felt the Silver Arrow punch through the chain-link fence as easily as a brick through a plate glass window, hesitate for a moment like a roller coaster right before a big drop, and then tilt forward as the whole massive engine began its horrifying dive down the hill.

The clackety-clack of the wheels went faster and faster and faster. Kate closed her eyes and felt a sick, weightless lifting in her stomach. She clenched her jaw tight and gripped the seat till her knuckles went white.…