PULL up the regulator and there's a massive whoosh as a sauna of warm steam envelopes your legs and a shower of rubbish flies out of the chimney and lands on your head.

But Sans Pareil is alive. She's moving. Ever so slowly, with great rushes of smoke and soot shooting out rhythmically as if she were a giant waking from a deep sleep.

It is a miracle. It has taken more than four hours for her 450 gallons of water to reach boiling point and produce enough steam to get her eight tons of rudimentary metalwork inching along the track at Darlington Railway Museum.

Sans Pareil is in Darlington for the next month before she heads to Wales for a televised bid to put right 170 years of hurt.

The original Sans Pareil - which means "without parallel" - was built in 1829 by Timothy Hackworth in his spare time in Shildon, County Durham.

He entered her in the Rainhill Trials, run by the Liverpool and Manchester Railway to find the best engine of the day.

She was one of the three favourites to win, along with George Stephenson's Rocket, built in Newcastle, and an engine called The Novelty from Sweden.

But The Novelty cracked a steam pipe on the first day of the trials and withdrew. And on the second day, Sans Pareil blew a cylinder, and withdrew, leaving the Rocket to win by default.

There have been cries of industrial espionage ever since because Sans Pareil's fateful cylinder was made by the Stephensons in Newcastle. Did George nobble the only competitor who stood between himself and a £500 prize and a place in history?

"Probably not," says Alan Pearce, curator of the Timothy Hackworth Museum, in Shildon. "Stephenson cast 24 cylinders and, unfortunately, Hackworth chose one with an air bubble in it.

"But if Sans Pareil had won, history would probably remember Hackworth in the way that it does Stephenson."

Sans Pareil's chance to re-write the history books comes in October on the Llangollen Railway with the re-running of the Rainhill Trials in front of the BBC television cameras. The replicas of all three engines will be there.

The Sans Pareil replica was made in 1980 by apprentices at British Rail Engineering Ltd (BREL) at the Shildon Wagonworks.

"This is probably the closest to the original of the three," says Alan. "The only differences are for safety reasons. It's got brakes, for a start, which weren't invented then, and a handrail to stop the driver falling off which was considered an unnecessary luxury."

The original was indeed such a safety hazard that men were employed to run alongside it, liberally dousing the coaches with cowfat or tallow to stop them catching fire.

"The coaches became so hot you could light your pipe from them, but one of the rules of the company was that any man caught doing so would be sacked," says Alan.

With its modern airbrakes, there should be no such problems as Sans Pareil makes her first visit to Darlington. She'll be giving rides to visitors from midday to 4.30pm tomorrow, next Sunday and on the following Sunday and Monday of the Bank Holiday weekend.

She'll be crewed by members of the Darlington Railway Preservation Society whose shed will also be open to the public next Sunday.

A ride on Sans Pareil comes in the admission fee to the museum, in North Road, which is currently reduced to 75p for children and OAPs and £1.50 for adults - and so a ticket for Sans Pareil is almost as cheap as it was in Hackworth's day.