Sergeant Colon and Corporal Nobby Nobbs, who had taken it upon themselves to patrol the line of expectant sightseers queueing for a ride on the train, looked around uncertainly. It had been a long time since Sergeant Colon had been a young man, and as for Nobby Nobbs, although it was generally agreed that he was the younger of the two, there was some doubt about whether the term Homo sapiens could be applied to him; the jury of Ankh-Morpork was out. Colon and Nobby were supposed to have been on the beat in the Shades, but Colon had delegated that task to a couple of new recruits. ‘Good experience for ’em, Nobby. And it’s likely to be a dangerous business, this streaming engine. Needs someone to have a look-see – a couple of experienced coppers, let’s say, prepared to put therselves in harm’s way for the public good.’

‘Young man … excuse me,’ came the voice again. The speaker was a harassed-looking lady with two boys at heel, who weren’t at all at heel and were expressing their frustration at having to wait for the promised ride on the train in the supremely annoying ways that only small children can manage. In a desperate attempt to distract them from their contest to inconvenience as many people in the queue in front of them as possible, their mother had seized on the first official-looking people who might be able to entertain her offspring with some interesting facts.

‘We were just wondering if you could tell us how this locomotive goes?’ she asked.

Fred Colon took a deep breath. ‘Well, missus, there’s the boiler, you see. It’s like a kettle.’

This was not enough for the smaller child, who said, ‘Mum’s got a kettle. That doesn’t go anywhere.’

His mother tried again. ‘And how does this “boiler” work?’

‘Well, you see, it sends the hot water to the engine,’ said Nobby hurriedly.

‘Right,’ said the lady, ‘and then what happens?’

‘And then all the hot water goes into the wheels.’

The elder boy looked sceptical. ‘Really? How’s that done?’

Nobby, cornered, said, ‘I think the sergeant can tell you that.’

A little bead of sweat appeared on Colon’s face and he was aware that the two children were looking at him as if he were some kind of exhibit. ‘Ah, well, the water is magnetic, right, because of all that spinning,’ he said.

The elder boy said, ‘I don’t think it works like that.’

But Colon was on a roll and ignored him. ‘The spinning causes the magnetism and that’s what makes the water stick in there. Lots of iron in train wheels, stands to reason. And that’s what keeps the train on the iron road, magnetism.’

The smaller boy changed tack. ‘Why does the engine go chuff?’