The rails gleamed and vanished into the black mouth of the tunnel. As always, the underground station stretched empty, the benches unoccupied, the platform swept, every tube of neon casting a uniform yellow light.

Then something stirred. David squinted as the destination board lit up. A row of orange letters, every pixel bright. The next train would arrive in precisely one minute, an express right into the heart of the city. Faraway, a rumble grew. The air gusted into a breeze, then a gale. Until with a cannon shot of noise, twin headlights shot from the tunnel, pursued by six rattling carriages that hissed and squealed to a halt.

A pair of doors stopped right in front of him. In their polished metal surface, he could see his silver-grey hair, his tense expression. Peering through the windows, he checked nobody was inside. The third carriage, the one he always chose. Not too close to the front and with three carriages to escape behind.

The doors slid open. Cautiously he leaned in. A long glance up and down, scrutinising the carriage from end to end. He checked the carriages either-side as well, at least what he could see through the oblong windows of the connecting doors.

In one swift movement, he stepped inside. Then paced to his usual seat halfway down. He sat gingerly, still observing. Watching every door, the platform beyond, ensuring he stayed alone.

Usually he didn't see anyone. But if he did, he stayed out of their way.

He could never become complacent. The city transit system might appear deserted, silently running itself, but there were others. Die-hards and chancers and desperados who prowled for whatever reasons. He didn't travel in often, only out of necessity. But he'd received a system alert — a vital component gone down — that he couldn't fix remotely.

The doors closed with a soft thud. Jolting, the train charged forward. Its motion was like an impatient child sprinting into a dark hallway, reaching its fastest speed, then slowing violently to a crawl. He was used to the stop-start motion, the short periods of nothing as the train shuddered through darkness. Pre-virus, he'd commuted on this line everyday, there and back like an automaton.

In a flood of new light, the train burst into the next station. Turnpike, he knew, without needing to look at the white letters on the blue bar on the curved station wall. Named after an ancient toll road that had once existed on the surface. Now there'd be nothing moving up there: no traffic, no people. Only blocks of empty flats and streets overgrown with weeds. Everyone had fled. Or else died.

Two years ago, the trains had always been squeezed full of people. Every seat taken and more passengers dangling from the handholds. The air fetid and unpleasant, with no space to read or breathe. Often he'd spent each journey staring at the dandruff on the back of someone else's collar. Never had he envisaged having a whole carriage to himself. Yet now he travelled in splendid isolation, only half remembering the claustrophobia of the past.

The train juddered to a stop. The doors facing the platform slid open. He watched them methodically, first the double and single on his right, then the double and single on his left, ensuring no-one else got on. No-one ever did, but that didn't mean no-one ever would. On previous excursions, he'd sometimes glimpsed eyes and shadows and his blood had run cold. Someone else scrutinising and checking. Everyone on their guard. Wary of the dreaded two metre encounter.

The doors closed, and the train jerked forward. He felt his heart rate slow a little, at least until he reached the next station where the whole repetitive process — doors opening, eyes scanning, doors closing — would happen all over again.

Nine stations altogether to his destination — and six more to go. The ones with interchanges scared him the most, where people from other lines might switch to his. Thankfully his route was direct. He didn't have to change anywhere. The stairs and corridors that connected lines were the most dangerous, full of dead-ends where it was impossible to escape anyone and the risk of a two metre encounter became almost a certainty.

There were no official statistics anymore: how many new infections there were, how many people had perished. Apart from technicians like himself, no one monitored anything. The system ran itself, the trains all driverless, the stations self-regulating, an almost perfect machine that pleased itself.

He clamped his hands together, trying to reassure himself. As long as he stayed alert, watchful, the chances of a fatal encounter were low. Yet even out in the greenbelt where he lived — safer than the city — outbreaks occasionally happened. Ordinary looking men and women who became carriers, initially with few visible symptoms. Fortunately the virus could only spread by close proximity, two metres or less of air. Not so much of a hazard in the wide outdoors, yet deadly down here in all the confined spaces. If you saw someone coming towards you, you fled. Into another carriage, onto the platform, out of the station, as far away as you could get.

The train slowed and stopped at the next station: Manor. No-one on the platform, nor at the three stations after. He prayed his journey would remain uneventful, his vigilance keeping him safe.

As the train neared Cross, he drew a deeper breath than normal. This station connected with three other lines. There was also a huge station concourse above. A much higher risk of someone boarding, as well as being infected.

To his relief, the platform stood empty. No-one boarded the train.

The doors closed and the train accelerated out. Only two stops left now: Russell, then Born, where gratefully he'd disembark.

Almost safe. Almost sound.

He heard a click. The hairs on his neck rose as he watched the connecting door to the next carriage slowly open. Sometimes paranoid people walked all the way through. Yet never into a carriage already occupied. Nobody in their right senses would do that. Only someone wholly unafraid. Or infected.

He tried to push himself up and run into the opposite carriage. His arms and legs froze though as he saw a shiny figure emerge. A homeless man wrapped in a space blanket, he thought at first. Or maybe a drug-crazed party-goer dressed like the Tin-Man. The figure glided at speed, not reaching for any of the handholds even though the train was swaying. On its head it wore a silver cap, on its lapel a round badge, and on its hip a slim ticket machine.

It was one of the conductors that roamed checking tickets.

He slumped back into his seat, feeling his heartbeat slow. Nothing to worry about. Just another piece of automation, a relic from pre-epidemic days when there had been too many fare dodgers. He'd heard these conductors still existed, though he'd not seen one in a long time, not like this.

He reached into his jacket for his ticket and held it ready. He couldn't remember what capabilities these conductors possessed, whether the thing would indulge in small talk or merely punch the small piece of cardboard.

Three feet away, the conductor stopped, bent its head forward, and flashed its eyes.

"Tickets-please!" Its voice was stretched wire.

He held his ticket higher — closer — wondering if it could see it properly. Perhaps its scanning equipment was broken, its processing software malfunctioning.

"It's a one day travel pass," he said for the sake of saying something, aware it might not even hear him. "Valid on any train. Valid at every station."

"At-what-station-did-you-board?" The conductor lunged and extended a silvery arm. With claw-like fingers, it snatched the ticket from him.

"At Fosters," he said, wondering why it had to know.

In a whine of servo motors, the conductor raised the ticket to its diode eyes. A flash of laser red as it scanned the code. Then with another whine, it lowered its caliper arm and thrust the ticket back. At the same moment, the train rushed into Russell station.

"Please-remain-seated-until-the-train-comes-to-a-stop," it announced as the doors opened. "Please-take-all-your-bags-when-exiting-the-train. All other passengers move down inside the car."

"There isn't anyone else. I haven't seen anyone at all."

He wanted to laugh at the thing's obsolete programming. Yet he tried to be polite, though it was only a machine. A two metre encounter, he realised, except it was made of metal and wires, thank God, not blood and germs.

"Please-mind-the-gap. Stand-clear-of-the-doors!" The doors closed and the train moved off.

David reached out to take his ticket back. He winced as one of the conductor's claws jabbed into his flesh. And turning his hand over, he saw the thing had scratched him, a tiny cut on his thumb that dripped blood.

"Ow! Look what you've done!" He pressed down a finger, annoyed. Machines weren't supposed to hurt people, especially not when their ticket was valid.

"I-am-sorry-for-any-inconvenience-caused." The conductor's voice grew plaintive. "Due to staff shortages, there may be a slight delay."

"You're kidding?" This time he couldn't help but smile. "I guess there's not much for you to do anymore, not since the epidemic. Perhaps you need to be reprogrammed, or even phased out. How many of you are there left, anyway?"

The thing didn't answer, but it stayed where it was, its arm stretched out like a sword. It was starting to spook him, and he wanted it to move away.

"You should see if there's any more passengers?" He waved his injured hand at the next carriage, knowing there was nobody else. His wrist was sore, his arm too.

"Please-change-at-the-next-station-and-take-the-next-train-home." The thing's monotonous voice spiked as if in triumph.

"What!" He stared at the conductor in surprise. "Why would I do that? I have a computer to fix."

"You-are-too-late."

"How would you know?" His arm throbbed, painful. Glancing at his hand again, he saw it was swollen and blue. "Has central control sent you?"

"This-is-the-end-of-the line. The-next-stop-will-be-your-last." The conductor's eyes suddenly glowed again, it's expression strangely human.

And with horror he realised his mistake. He'd been careful, but not careful enough.

"What have you done?" he said weakly. "You've injected me — with the virus?"

"You-are-the-virus." The conductor shook its insect head. Then swung its arm back down and slid away along the carriage. "Have-a-nice-day."