The rumbling hadn’t increased, which meant it wasn’t a train coming their way after all. A silly thought made in panic, Karl realised. But the rumbling sound persisted, and they moved onwards slowly, fearful of what might lie ahead.

They turned a corner, and stopped dead at what they saw ahead. Liz grabbed his arm.

‘My God, you’re right. A train. There must be a way out.’

He didn’t share her glee. The train was just an outline in the dark, with nothing to illuminate it. No headlights, no dashboard dials or switches. It sat there dark and dead, and Karl soon realised why. It was a carriage, not a locomotive. Nothing to power it, although he could still hear that soft rumbling coming from somewhere. There were a number of short, cylindrical shapes arranged before it. They’d stumbled across an abandoned carriage, and it blocked their way ahead.

‘Oh my god, it’s a station. We can get out.’

She released his arm and stumbled ahead. To the left side of the carriage, his eyes made out the edge of the seemingly endless wall on their left, and then a void beside the train. As they inspected their surroundings, Karl realised that the station was nothing but a widening of the tunnel. The platform was simply a stone shelf. No old vending machines, no ticket booth, no turnstiles. The roof was higher; the walls were flat rather than curved. He couldn’t see a doorway. He started to lose pace. Something wasn’t right here. And the rumbling continued.

‘Come on,’ she called back, getting further ahead. ‘We’ll be free soon.’

‘Stop,’ he shouted. She ignored him.

But Karl’s jog became a walk as his eyes started to make out shapes on the platform. Small poles rising up to platforms. Chairs and tables. And some kind of underground chain-link fence, blocking their path.

Seeing it too, Liz halted. He stopped by her side, and she clutched his arm.

‘What’s going on?’ she said, her tone one of dashed hope.

‘I heard about this place,’ Karl replied, turning away from the fence to inspect the dark shapes before them. Beer barrels, attached by pipes to pumps – the source of the rumbling noise – on the side of the train. ‘We’re going to get out, Liz. This is Banker Avenue Line train station. It’s now an underground bar. The Apocalypse.’

He started walking, but felt resistance on his arm. Liz hadn’t budged.

‘Why are there no lights?’ she said. ‘Maybe it used to be a bar and it’s abandoned as well.’

He walked on and, although her grip on his arm was lost, he heard her feet on the stones behind him. They threaded their way between empty barrels and stopped just feet from the end of the carriage. There was a set of wooden steps leading to a door. Karl went up and put his face to the glass. He’d already decided that it would be easier to bust the door than to fight past the chain-link fence.

‘Fridge lights are on. God, I could do with a beer right now.’

‘Is it open? Quick!’

He turned the handle and pushed, and, beautifully, the door swung inwards. Immediately, Liz was up against him, pushing, desperate to get in.

‘Liz, Jesus, slow d—’ He was halfway to his feet when he heard a series of thumps in the bar, getting closer and closer. In the blackness he couldn’t fathom the direction, so he turned to where his back had been facing because that was his vulnerable side, and held up his arms to protect himself.

A moment later a train smashed into him.

Best guess: suspension in the encoding process of his frontal lobe, or however it worked. He’d spent so long immersed in the underground railway that a strange noise in the dark had fired-up a connection to trains. But he was no longer in the tunnel. And it was a disused railway. So, a half second before he was slammed into a wall, logic reassessed what had slammed into him: not a train, but a person.

The proof came in the next instant: ‘Going nowhere, arsehole.’

He felt a knee jam into his stomach. As he doubled over in pain, his brain thought, double-leg takedown. Just another memory association, but this time a helpful one, based on his love of watching combat sports. You wrapped your arms around the opponent’s hips, lifted, twisted, and dumped him hard onto his back. But academic knowledge was a far cry from pulling the moves in reality.

He jerked, but the big mass in his grip didn’t move more than an inch off the ground.

He felt something hard ram into his backbone, probably the guy’s elbow. A twelve-six elbow strike, highly illegal in combat sports. Pain spread like cracking ice throughout his body, sending his left arm numb. He still couldn’t get his breath from the knee strike.

A heavier blow landed, not as sharp this time. Two fists crashing down on his shoulders, accompanied by a grunt of exertion. The next pain was in his forearms, elbows and wrists as he was driven down, hard, onto the floor. He rolled, curling into a ball to protect his cramping stomach from further injury, one arm tucked against his abdomen and the other against his exposed head in case the guy’s next tactic was to drive down a boot. Illegal as heck, but the rules weren’t in play here.

Instead, he heard pattering footsteps. Then there was the sound of a scuffle, body on body, and a second later a screech of pain.

‘Karl!’

A hand hit his head, but just a soft blow. Someone feeling about in the dark. Then it was back, touching his hair, latching onto his collar and pulling him up.

Liz, he realised. The man, nearby, was still yelling. Karl got to his feet, grabbing hold of Liz’s hand. He stumbled towards a thin vertical line of light that he hoped was a way out.