The noise was loud enough that any living thing within close proximity would hear them coming. Several strained heartbeats later, the noise vanished, and he slipped through the gateway. The main corridor was similar to the one that led to the Biosphere facility. It had been drilled with a “mole,” or tunnel-boring machine. Only a TBM left behind such smooth walls.

Overton checked the tunnel. His HUD again blinked clear of heat signatures.

“It’s clear,” he said. A moment later, Sophie caught up with him.

With a nod she paced down the passage, shoulder to shoulder with Overton. At some point she’d grown rather fond of the brusque Marine. Without realizing when it had happened, they had become a team.

The image of an idle train filled their displays as they rounded the corner. It rested on the tracks like a sleeping beast. Overton stared at the relic from the past; it had been years since he had ridden on a train. A pair of red emergency lights glowed above the front car door, illuminating the symbol of the NSA. Overton approached the train, jamming his combat knife between the car doors.

“Help me,” he said. Working together, they pried the door open and slipped inside the empty train. It was littered with debris—rusted metal fuel barrels, a pair of rubber tires stacked neatly on top of one another, and a plastic container filled with tools.

“Looks like someone’s used this train since it was decommissioned,” Overton said.

Sophie shrugged, making her way past the barrels and into the conductor’s car. She guessed several airport employees who knew about the facility would have brought their families to the tunnel during the solar storms. She’d harbored a faint hope that they might have done so again, but she saw no sign of recent habitation. She checked her HUD for a radiation reading. It was fairly high—high enough to keep survivors out of the tunnel. There was little chance of running into any humans.

Overton pushed his way into the tiny car and peered over Sophie’s shoulder, studying the gears. “Well, do you know how to run this thing?”

Sophie pushed a red button on the middle of the console, and a row of lights flickered on. He patted her on the back and slipped back into the second car, taking a seat on one of the dusty benches. His stomach was growling and his throat was parched. It had been several hours since he’d had any nutrition, and a couple seconds without his helmet wasn’t going to kill him.

“Hold on to your seat, Sergeant. If I’m correct, this train can reach a maximum of three hundred miles per hour. That should put us at White Sands in a little over two hours.”