She didn’t notice the suitcase until the train was pulling out of Piccadilly Circus. It was a pine-green, hard-shelled case with wheels and an extendable handle, pushed up against the other side of the glass panel. How long had it been there? At Charing Cross, she looked to see if somebody claimed it. People jostled past it on their way out, irritated by the obstacle. A small woman with a scrappy high ponytail banged her knee on it and let out a bleat of pain. The woman scowled around for the owner, but no one came forward.

Almost as soon as the train had left the station, it stopped. Tamsin looked at the suitcase. Then through the window behind her at the tunnel blackness with its dirty arcana of wires and pipes. Then back at the suitcase. Someone was watching her: a tall boy about her own age, with broad shoulders and something slightly Asian, Chinese or Japanese maybe, about the eyes. He was standing in the middle of the carriage, holding the handrail with both hands, elbows flexed as if about to do a chin-up.

The boy nodded towards the suitcase.

‘Is that yours?’

Tamsin shook her head. ‘Is it yours?’ she asked, stupidly.

‘No.’ The boy leaned forward and tapped the shoulder of an older man in a pale grey trench coat. ‘Excuse me, sir: does that case belong to you?’ His voice was respectful and refined, the accent upper class without a hint of arrogance.

The man frowned, shook a no, turned back to his paper.

Tamsin and the boy each read the same thoughts in the other’s face. When the train jolted, they both jumped. But it was just a false start; another jerk and they were moving again. Tamsin looked away, suddenly sheepish. No one else seemed concerned about the case; surely they were both being paranoid.

But her gaze was drawn back. Tamsin’s mind played forward to the blast, the train carriage crumpled like a Coke can. Though of course, she wouldn’t see that. She was sitting next to the suitcase; she would be killed outright. Tamsin Jarvis, daughter of the conductor Bertrand Jarvis, was killed outright in the attacks of 25 November. Unless the pane of glass was thick enough to protect her, just to begin with. Maybe it would shatter, or melt down onto her, sticking her clothes to her skin…

At Embankment, the same thing again: one lot of passengers shuffling off, the next lot starting to push their way on prematurely. And all the time the suitcase squatting there, unclaimed. At the last minute, Tamsin stood up and burrowed to the exit. Two seconds later the tall boy followed her out. Neither of them said anything.