Luke moved along the side of the carriage’s blackened, spring-loaded bogey until he came to the first gap big enough for a man to crawl through. The doors to the carriages above would be locked, he was sure of it, but down here in the darkness there was plenty of space to hide. Luke took out the pencil torch the police had lent him, held it against his sleeve to muffle the sudden light, and switched it on. He crouched and squeezed through the space between the wheels, hearing his breath coming in short gasps. Very slowly, avoiding any sudden movements, he swept the space beneath the carriage, playing the torch beam across every surface, probing into every recess and crevice.

It took him nearly two minutes to search the length of the undercarriage and that was just one car. He moved on to the next, noticing his hands and arms were already black with grime. After searching below six carriages with no sign of Squires, Luke began to feel failure creeping up on him. He was deep inside the station now and he just didn’t think Squires would have come this far. Perhaps he had misjudged him after all. Maybe the bastard had indeed made a run for it and was already flagging down a cab and heading out of London right at this minute. He crawled out from under the carriage, scraping his knee painfully on the stones, and stood still, listening to the sound of his own breathing and trying to ignore the throbbing pain in his shoulder from the shotgun blast. It had grown worse in the last hour and if he didn’t do something about it soon then the wounds would become infected. He was running out of time.

Retracing his steps, Luke paused by the first carriage, barely daring to breathe. You bloody idiot, Carlton. You forgot to search beneath the locomotive. But if Squires was down there, he was cornered. And that made him desperate and dangerous. Unarmed, Luke would have to improvise. It took him a few seconds to find what he was looking for: a stone with a sharp point. Crude but effective. With this in his right hand and the torch in his left, he ducked down beside the engine and squatted beneath it. He had the torch switched off and now he almost held his breath, keeping deathly still, waiting, watching and listening.

And that was when he heard it.

It was just the softest clink of stone against stone, barely audible above the growing hum of pre-dawn traffic from Euston Road. But it was a noise unmistakably made by a human. Someone was down there.