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.

The coupling between the fourth and fifth cars had a different problem: instead of being stretched, it was suddenly and violently compressed. The buffers absorbed some of the impact but the rest was transferred into the cars. Something had to give, and the back wheels of the fourth carriage went off the track, followed by the front wheels of the fifth carriage as the two freight cars jack-knifed into the side and mouth of the tunnel.

The rest of the train ploughed into the now-derailed carriages at full speed.

From my point of view I just heard a massive grinding, booming crash, something like a gigantic set of dinner plates being thrown down a staircase, and I felt a shudder through the wheels and metal of the train. Dust and debris went flying, along with shattered bricks from the tunnel mouth. And then just that quickly it was over, and the carriages kept rolling, carrying us away down the tunnel. Anne and I were left in the darkness, riding the back end of the suddenly much shorter train.