Then Iron Girder snorted steam, panting like a dog ready to be unleashed. Moist took a deep breath, stuck his fingers into his jacket and smiled with a confidence which had blossomed just a second before when he had finally heard the subtle sound he had been waiting for.

‘It’s a little-known fact, my friends, that these fogs have remarkable solidity. Allow me to demonstrate.’

He stepped off the edge of the cliff beside the track and stood there with the fog swirling around his ankles. He heard gasps from behind. He turned and faced his fellow travellers with a huge grin and a silent sigh of relief before stepping back on to what might be called solid ground.

‘You see. Would you like me to run to the other side and back again while this mystical phenomenon continues, as I believe it will, or shall we all go over now, while the time is ripe?’

‘D’you mind if I try?’ said Vimes.

There was a twinkle and Moist said, ‘By all means, commander.’

And Vimes disappeared into the swirling fog, lighting his cigar, saying, ‘Just like standing on a pavement. Amazing. I suggest you make steam, Mister Simnel! I’m in some doubt about how long such a, as you say, mystical phenomenon will last. So I think alacrity is our motto here, gentlemen.’

Simnel, resisting the natural temptation of a scientist to examine the phenomenon more closely, looked around and said, ‘Oh aye. All aboard, everybody!’ And after a moment, he added, ‘Quickly … please.’

Moist looked at Simnel and said, ‘Do you now believe, Dick?’

‘Yes, Mister Lipwig.’

‘But do you really believe?’

‘I surely do, Mister Lipwig! I believe in the sliding rule, the cosine and the tangent and even when the quaderatics give me gyp, yes, I still believe. Iron Girder is my machine, I built ’er, every last rivet carefully forged by ’and. And I reckon if I could bolt rails on to the sky, Iron Girder would take us to t’moon.’

Moist whistled and heard a signal from below. He raised his voice and said, ‘Forward, please, Mister Simnel!’ And immediately there was the familiar chugging sound of a train anxious to be travelling and getting up steam. Moist loved the moment as the power built up slowly and by degrees until there was a rolling thunder, taking charge of the universe, and they were moving into the villainous fog and on to the bridge.

It was difficult to see anything from the footplate, but Moist could just make out Simnel’s white face as the vibrations and swaying intensified. Despite Moist’s dramatic demonstration earlier, he could tell that Simnel and his crew were terrified and even he began to doubt whether the bridge would in fact hold under the pressure. And then the vibrations suddenly ceased and there was a strange sensation as Iron Girder left the rails and she flew.

Down below, the fog curled into even stranger shapes, spiralling vortices, stirred up by the passage of the train, and after a strangled few minutes there was a thump of wheels on rails as Iron Girder consented to exchange flight for the sensible permanent way once more and then Dick blew the whistle and kept blowing, and she was bowling along again as if nothing uncanny, mystic or even eldritch had happened.