Anne turned and ran, crossing the gap from the fourth to the third carriage with a running jump. I gave Will’s group a last glance and followed, putting a little extra speed into the leap to make sure I cleared it. Up ahead I could see a dark wall with streetlights running along the top of it, and the tracks ran into a black circle. We’d had a long run in the open but London’s a built-up place, and every railway line in the city goes underground sooner or later. In this case sooner or later meant now. As soon as I’d made it onto the third car I dropped and craned my neck back to look.

Will, Lee, and Captain America were staring at us through the forcewall, watching the tunnel entrance draw nearer. I wasn’t close enough to see their expressions but I saw the jolt go through Captain America as he figured it out, and as the tunnel mouth swept over us he yelled a warning and turned to run. Lee followed; Will stayed a little too long and had only just started to sprint away as the fourth carriage passed into the tunnel, and through the shrinking window of the tunnel mouth I had one last glimpse of their backs before the forcewall hit the top of the tunnel and physics took over.

I didn’t get much of a view of what happened next. It was all very fast and my attention was focused on hanging on. Looking back on it with the benefit of hindsight and having read the accident reports, what I think happened was this.

The forcewall hit the brickwork at the top of the tunnel and, just as it had with the bullets, the forcewall tried to transfer the momentum of the impact down into the freight car. The forcewalls I use are powerful by one-shot standards but there’s a certain maximum amount of momentum they can transfer. This doesn’t matter in normal situations because there’s no way a normal human can break the wall’s limits. The momentum in the sixty-thousand-pound freight car fell well outside the wall’s limits. The wall resisted for a fraction of a second, then vanished as the energy stored in the discs ran out.

But in that fraction of a second the wall transferred enough momentum to put a major dent in the freight car’s velocity, and all of a sudden the fourth car was moving at a different speed from the rest of the train. The freight car wasn’t harmed, at least not directly—the discs spread the momentum through a large enough volume that the car got away with only minor buckling. The coupling between the third and fourth cars didn’t do so well. It had been designed to withstand gradual stress, not instant stress, and it snapped, separating the first three cars and the engine from the rest of the train and sending a massive jolt through the metal.