The tugging on the cabin door startled her. Perhaps she had nodded off.

The door seemed to have stuck in place and took some force to shift. It opened with a crack of splintering ice, letting in chill air and a swirl of snowflakes. A burly man carrying an oil lantern clambered up a ladder to the doorway. Aged maybe forty, he wore the uniform of the Iron Way guild, a subdivision of the ancient House of Beavers, the Wall engineers. He held up his lantern. ‘Are you all right, madam?’

She forced a smile. ‘Not exactly. Cold. Bored. Hungry! But alive, as you can see, and keeping myself warm. What’s going on, officer?’

He rolled his eyes. ‘You name it. We’ve got drifting snow in some places. It fell too thick for the blade, and we had to change that for the midwinter design, and in places we’ve had to send the lads out ahead with shovels to clear the line. Then there’s trouble with the boiler, external lines freezing up and splitting. Sorry for the delay.’

‘Don’t apologise. You’re stuck as well. As long as we get there in the end.’ She glanced through the window. ‘Where are we, by the way?’

‘On the Wall. But still well to the east of Etxelur. We’ll get there, miss, you’ll see.’

She frowned. The Wall was inhabited from end to end. ‘But it’s still snowing. What District is this? There must be places we can shelter for the night. Maybe we should disembark and—’

‘We’ll get there,’ he said with some determination. ‘The engines of the Iron Way always get through. That’s what we live by, miss.’ He passed up a glass bottle of water, and, apologetically, a wooden bucket. ‘The latrine system’s seized up,’ he said.

She took the bucket. ‘Keep safe, officer.’

He was already closing the door. Listening hard, she heard his heavy tread through the snow, and then his rap on the next cabin in the line. But if he got a response, she couldn’t hear through the rushing of the wind. After that there was only the cold, and her candle.

She felt a draught. Since the officer had forced the door it wouldn’t fit back in its frame.

Up on the Wall parapet above Engine Seventy-Four, Thux was struggling.

It felt like he’d been out here for hours, and maybe he had been, for it was getting darker and darker. He had a lantern fixed into a prepared socket on the growstone surface, but all he saw by its light was a bubble of snowflakes, flying horizontally. The wind was coming from the north, flat and hard and muscular, and it sucked every bit of warmth out of any exposed skin, out of his cheeks, out of his hands when he flipped back the clever little lids on his mittens to expose his fingers for the finer work. And though the snow wasn’t settling up here, except in the engine’s vents and chimneys, the wind was depositing ice on every surface, on the growstone under his feet, on the pipework and handholds and rungs he had to use to clamber about.