When the thunder had died away, Detritus yelled up to him, ‘Hey, Mister Lipwig! Dis van is goin’ backwards! Der gritsuckers knew jus’ where to take der engine off!’

Moist turned and saw, to his horror, Iron Girder pulling away at speed from the now stranded guard’s van. He looked down at Bluejohn who was holding a grag in each hand and there was screaming as he banged their heads together and tossed them into the darkness between the tracks.

‘We’re going backwards, Bluejohn!’ yelled Moist. ‘Take us forward, could you!’

There was a jerk as Bluejohn stopped the guard’s van dead, quite possibly with his feet, and Moist jumped down on to the shuddering flatbed.

‘Nice work, Mister Bluejohn. Now get out that thing Mister Simnel’s lads made for you, please.’

In his curiously childish voice, Bluejohn said, ‘Oh yes, Mister Lipwig, I can do dat and I can tow der guard’s van as well.’

Vimes dropped down from the roof where he had been making life difficult for the grags on top – who were now essentially on the bottom – shouting ‘What the hell’s going on! Why’ve we stopped and where’s the rest of the train?!’

‘Those buggers have uncoupled us!’ yelled Moist. ‘But it’s no problem … there’s a handcar on Bluejohn’s flatbed … for emergencies!’ And, indeed, when the pedals of the handcar started to turn, the guard’s van accelerated and shot like an arrow towards the disappearing Iron Girder.

Bluejohn’s big face was aglow as he pedalled like, well, Bluejohn, because nobody else could have made that flatbed fly along the rails. It rattled and screeched and complained, but the troll’s huge feet oscillated up and down in a blur and the inner demon of Moist von Lipwig whispered to himself, ‘A little treadle machine to help somebody travel fast? Might be an idea to remember that.’

The whistle of Iron Girder echoed around the canyons, and Vimes shouted, ‘Get me up close to that train, officer!’

Trolls don’t sweat as such. A kind of blooming takes place instead. Bluejohn grunted, ‘Gettin’ a bit outta puff now, commander … but I’ll do my best.’

Bluejohn’s handcar, still dragging the guard’s van, including the recumbent grags, slammed into the last carriage, and before it bounced away again he reached forward and grabbed a buffer in each hand. Immediately Vimes flew like a demon, running across Bluejohn’s ample back and into the besieged carriage. Moist followed as best he could. Grags and delvers were everywhere, still trying to get into the armoured carriage ahead, and then it was just a matter of who was friend and who was foe, and there were far fewer friends, making it very easy to spot the foes.

‘Come on, lads! No speeches, you sons of mothers!’ Vimes shouted back to the others in the guard’s van. ‘You know who the enemy is and you know what to do … Get them before they get you and don’t let them get anywhere near the King! I’m heading up on the roof!’