The rail was finally free. Eight men, stripped to the waist, carried it away with pairs of giant pincers, and brought a new one, whole and gleaming under the yellow lights. They lowered it into place with brief, shouted commands and started to knock it in, fixing it back to the sleepers with rhythmic blows of their lump hammers.

Dalip watched them and envied their nonchalant expertise. Oh, it wasn’t like he was going to spend his life fixing broken rails – he was going on to make trains that floated above, rather than ran on, rails – and this was just a placement, the first of many, to give him some idea of what engineering was supposed to achieve at the sharp end. How the whole infrastructure of the Underground – tunnels, trains, ventilation, pumps, stations, even the movement of people from above to below and back again – had been designed and built.

There was so much to learn, he despaired sometimes.

Stanislav carried the first part of the rail welder to the site of the join, dumped it by the side of the track and jerked his head to indicate that Dalip should follow him.

He did so, obediently, like he did everything else asked of him. There was a shovel, an oxyacetylene torch, the gas tanks to go with it, and a reaction vessel with an outside so burnt it looked like a cinder. Bags of dust. Wet sand to seal the casting. All of it needed to be moved.

It was hot enough in the tunnel already. It was the hardest work Dalip had ever done, and he was barely an hour through. The thermite reaction they were setting up would fill the tunnel with acrid smoke and thick yellow flames, making the harsh conditions worse, and yet these men, hard-muscled and terse, laboured in it day after day. He couldn’t cope. He’d faint and fail. He didn’t belong there.

And whether Stanislav had spotted the panic in the boy’s eyes and realised he needed reassurance, or whether it was simply well-timed: he clapped Dalip on the back of his orange boilersuit, hard enough to rock Dalip on his feet, and gave him the thumbs-up.

It was enough to steady his nerves. He was here to learn, not to be humiliated.

The whole tunnel shook as if struck. Dust hazed the air, and the lights flickered. The whole work crew stopped in mid-swing.

‘What—’

Stansilav put an oil-smeared finger to his lips, and listened to the noises with wary attention.

They were a long way under London, and there should have been nothing else down with them but other tunnels. If something had happened above, on the surface, it would have had to have been immense to reach them. A bomb? A building falling down? An aircraft crashing? All three?

There was nothing to compare with the initial concussion, though a low groan of pressure creaked through the walls.