He patted Moist on the shoulder and said, ‘Mister Lipwig, I’m sure there’ve been times in your life when you’ve seen a wonderful opportunity to steal something valuable and you’ve decided for various reasons not to? Well, I feel as you would then. This place is a sink. Who knows what dreadful things are happening behind closed doors.’ He shrugged and went on, ‘But you can’t kick down every door in the universe. And we have more pressing matters to attend to.’

Moist accepted this sad explanation and after a fruitless hunt around for their assailant they turned their backs on the Paps to head back to the station. As they left the lookout point, they heard a train whistle in the distance. Far away, climbing into Zemphis on the main line from the Plains, there was a streak of scintillating sunlight with a trail of steam.

Vimes looked at Moist and said, ‘What the hell is that? There isn’t another train due today, is there?’

‘Well, Dick said he was polishing up Iron Girder for the big event, and before we left he was talking as if his favourite locomotive had been getting a complete overhaul. That must be her.’

In fact, it wasn’t just an overhaul. When Moist had shown Simnel the micromail he had received as spoils of the battle with the dwarfs at the Quirm railhead, the young engineer had smiled and said, ‘Aye, I know what the secret is here. This is a metal stronger than iron, malleable and half its weight, and it never rusts. Its ore is rare but it’s the base of a new alloy I’ve made. I call it sorortanium, which Mister Thunderbolt says means the sister of iron. It’s stronger even than steel! What boilers I could make with it if only I could get ’old of enough of the stuff! Thank you. It’s amazing and I know just what to do with it.’

As they watched the astonishing locomotive dealing with the steep haul into Zemphis, Moist noticed that the engine appeared to shrug off the load behind her. The Flyer they had come in on earlier had been creaking as it chugged up the last steep haul into Zemphis. This new train didn’t even seem to notice the gradient.

Vimes smacked his head and said, ‘Is that really Iron Girder? Last I saw of Iron Girder, she was providing a playground entertainment for grown-ups. If that’s Iron Girder,’ he said, pointing at the shimmering apparition, ‘surely she’s grown?’

‘It is Iron Girder,’ said Moist. ‘Dick spends all his time tinkering with her, over and over again, improving her at every turn and opportunity. And at the end, well, she’s still Iron Girder. She’ll always be Iron Girder.’

‘But she’s so easily spotted, all that shining … People are going to see her coming for miles! No chance of an inconspicuous departure, then!’

‘I know,’ said Moist. ‘But since you can hear her coming for miles, Dick said it won’t make any difference. Everyone’ll know we’re coming.’ Though the silver carapace in fact could make a significant difference in other ways, he thought.