The back of her green coat was just a few yards in front of him as they approached the station entrance. It would be easy to lose sight of someone here, but he was careful to stay close and allowed himself a little smile of satisfaction when he saw her turn abruptly off the main road and hurry down the tarmac slope. It seemed they had a train to catch.

A covered footbridge was the only access to the far platform and Naysmith paused, waiting until she was all the way across, before walking onto it and looking out on the station below. There she was, making her way down the long ramp that led to the curved platform, already lined with a number of early-evening commuters.

He checked his wallet for cash – he knew better than to risk using a credit card on a journey like this – when the rattle of an arriving train made him look up.

Mustn’t lose her now.

He hurried across the bridge and down the ramp towards the platform as the other passengers were boarding. It was a short train – only two small coaches – and he just had time to read the destination Severn Beach before leaping aboard through the last open doors. She was sitting with her back to him, at the opposite end of the carriage, so he slid quietly into a seat near the door and calmed his breathing as the train began to move. He gazed out of the window as Clifton Down station slipped away and they crept into the darkness of a long tunnel.

The guard appeared through the connecting door from the other carriage and began to make his way along the narrow aisle, checking tickets. In the fluorescent gloom, Naysmith frowned. He didn’t know which station she was going to. Taking out his wallet, he fished out a ten-pound note and held it ready for the approaching guard. He remembered the destination he’d read as he ran along the platform.

‘Return to Severn Beach, please.’

He could always get off sooner if she was going to an earlier stop.

The guard took his money, tapped a few buttons on a shoulder-slung machine, and printed out two tickets. After counting out the correct change, he walked back towards the other carriage, swaying slightly as the train emerged from the tunnel, daylight bursting in through the windows.

Naysmith blinked and looked out at the bright green foliage whipping by as they joined the river winding its way along the tree-lined Avon Gorge. When they slowed for the first station, he positioned himself so that he could see the back of her head between the seats but she made no move to get up. He settled back into his corner and stared out at the expressionless faces of the people on the platform, then closed his eyes. It had been a long day.

He found himself thinking of all those other faces, still so clear in his memory, each one a challenge, each one a reward. He understood the game now, knew why he played it, what it had given him. Casting his mind back, it was difficult to remember how he’d felt before it all began. He was different now. The game had changed him, altering something deep inside so that he couldn’t empathise with his former self. But there was no regret in that.

He felt the train begin to move. There was a change in tone as they rumbled over a bridge and he opened his eyes again. To the left, the Avon was broader, its sloping banks silted with grey mud. He wondered how far they had to go.

Nobody got off at the next station, but as the train pulled into Avonmouth most of the passengers began to get to their feet and collect their bags. From his vantage point, Naysmith watched intently, but she stayed in her seat, gazing out at the sheltered platform, its back wall decorated by a huge children’s mural.

The doors closed and they began to move once more, clattering slowly over a level crossing and following the single track as it curved steeply round to the right. The train passed in the shadow of an imposing old flour mill that towered like a derelict monument above the other industrial buildings lining the side of the track. There were no more houses now, but vast wind turbines could be glimpsed in the distance, along with cranes and mountainous piles of coal.

‘Any passengers for St Andrews Road?’ the guard called from the connecting door. ‘Request stop only, St Andrews Road.’

A request stop? Naysmith craned his head to peer between the seats. He hoped she wasn’t getting off here. Any station that operated by request didn’t sound as though it saw many passengers, and it would complicate things if he was the only other person to alight there.

He peered between the seats again but she sat still and quiet as the train coasted through a bleak area of warehouses and railway sidings. They rolled through the deserted station without stopping. Sinister-looking chimney towers belched pale fumes into the sky, but eventually even the industrial buildings became less frequent, and Naysmith felt slightly surprised as he realised he was gazing out at one of the Severn Bridges and, across the dark water, the Welsh coastline. Where did this girl live?

And then he felt the train slowing. The remaining passengers began to move, gathering their bags and getting to their feet as the guard called, ‘Severn Beach. Last stop.’

She was standing by the doors at the far end of the carriage, staring out of the window with the unseeing eyes of a tired commuter. Naysmith waited until the doors opened, letting her disembark before he got to his feet and followed.

A chill breeze greeted him as he stepped off the train and he thought he could smell the sea, a faint tang of salt on the air. Severn Beach station was little more than a single long platform between two tracks, one side almost lost in a tangle of overgrown weeds. He walked slowly by the solitary metal shelter and passed the corroded buffers that marked the end of the track, the idling hum of the train dwindling behind him. Ahead, he saw her walking down to the road and turning left. He quickened his pace a little. The platform opened out onto a quiet residential street – old and new houses huddled close to the pavement – a bleak little village on the edge of nowhere.

He turned left and walked along thoughtfully, some fifty yards behind her. It felt like somewhere that old people would come to – a quaint little tea room on one side of the street, bungalows with immaculate gardens, Neighbourhood Watch signs in windows. Ahead, he could see a steep tarmac slope that climbed to what looked like a seaside promenade, but his attention was on the figure in the dark green coat as she followed the road round to the left and disappeared from view.

When he reached the end of the road, he caught sight of her again, but elected to walk up the slope rather than follow her along the pavement. He quickly climbed the few steps up onto the top of the sea wall, and was suddenly buffeted by the wind. Before him, the vast grey expanse of the Severn rippled out towards the horizon, the bridge stretching away into the distance on his right. He turned away from it, pulling his jacket close around him against the cold as he walked along the promenade, his eyes following the figure on the street below as she made her way along the line of waterfront houses and turned down a small cul-de-sac. He watched as she unlocked her front door and went inside. It was a nice house – small, like most modern houses, but with its own driveway and a little patch of lawn. Smiling to himself, Naysmith walked on and followed the path down onto the beach.

The train back to Clifton Down was almost empty. He hadn’t realised how dark it was getting until he stepped aboard, the harsh interior lighting making it almost impossible to see anything outside. His eyelids were suddenly heavy, and he yawned before settling back into the seat. There was still a long drive ahead of him, but it had been a rewarding day.

He’d walked past her house on his way back to the station. There was a light on upstairs, and the hallway was illuminated, but otherwise the place was in darkness. He’d noted the small car on the driveway, the cheerful lace curtains, the plaster animals arranged on the doorstep . . .