Rarely has an engine been greeted with such rapture. Hundreds crowded the little country station or jostled for a lineside photo spot as the gleaming new locomotive cruised into the history books and the affections of the thousands who had paid for it.

Tornado, the first steam engine to be built in this country for almost 50 years, has completed its trials. Tomorrow it will be the star at a three-day gala celebration of steam on the Great Central, a preserved line near Loughborough in Leicestershire, where thousands are expected to flock to see the new star before it moves to the National Railway Museum in York for final testing.

The engine has taken 18 years to build and cost almost £3 million. An almost exact replica of one of Britain's last and most powerful postwar engines, this new 4-6-2 A1 Pacific brings back to life an apparently vanished class: the last of 49 locomotives built in the late 1940s was scrapped in 1966. A generation later, thanks to thousands of volunteers, sponsors and engineering firms around the country, an A1's firebox is again ablaze.

Tornado completed its preliminary trials at a stately 30mph. Soon it will begin trials at 60mph and faster on the main line. It will then be free to unleash the full energy of its huge firebox and haul chartered trains on Britain's rail network at speeds of up to 90mph.

As it is new, and has a tough German-built boiler, none of the speed and safety restrictions that limit older preserved engines apply. Like its forebears that thundered along the East Coast line from King's Cross to the North East, Tornado could run at up to 100mph, though most of those revelling in the weekend steam charters may prefer to take longer to arrive.

At 30mph, Tornado feels like a tiger straining at the leash. Its pipes, dials and levers are not yet encrusted with soot and oil, its footplate is unscratched and heat spews from the pristine firebox every time the heavy metal flap is hooked open. A specially adapted tender carries an extra 1,000 gallons of water - more than the Pacifics did when huge water columns filled thirsty engines at main stations. All have now disappeared from the network, together with the troughs between the rails that allowed engines to scoop up water as they went along. About one-and-a-half tonnes of coal capacity have had to be sacrificed, but the extra water allows Tornado to run for a further 25 miles.

On its first passenger-carrying run, Tornado performed faultlessly. Martin Ashworth, a volunteer driver on the Great Central, eased back the regulator and blew the whistle, while Roger Howes, the fireman, shovelled huge lumps of coal into the inferno. Hauling a full rake of historic coaches with hundreds of passengers who had contributed to the trust that built and owns the engine, the train gathered speed effortlessly. The low, throaty exhaust from the funnel settled into the familiar chuff once recognisable to every passenger and schoolboy enthusiast.

“She runs beautifully,” Ashworth said, staring down the line through the narrow front window at the side of the cab. On weekdays he is at the driving wheel of a heavy-goods vehicle; at weekends he drives the Great Central's engines ten miles along one of Britain's few preserved stretches of double track. “I swap 44 tonnes for 150 tonnes,” he said.

Howes, a retired policeman, is the swimming coach for Oxford University. His stamina belies his years: there can be few more exhausting jobs than shovelling coal into a firebox every three minutes. As he wiped sweat from his blackened brow or hosed down the dust on the footplate, you gained an idea of the sheer physical effort and strength it took to keep an A1 Pacific in full steam all the way to Newcastle.

When the project was launched in 1990, more than 1,100 blueprints by Arthur Peppercorn, original designer of the A1 Pacific - Indian ink drawings on linen that were stored in the National Railway Museum in York - were scanned and catalogued. The frame plates for the engine - the skeleton that determines a locomotive's personality - were cut from a single piece of steel and moved to a workshop in Darlington in 1994. It was hoped that Tornado would be built by the millennium. But with every component having to be machined and moulded individually - with the generous support of engineering companies nationwide - and relying on volunteer labour, Tornado took far longer to build than planned.

The 6ft driving wheels were cast in 1995 and the cylinders unveiled a year later. But the boiler, the greatest challenge, took much longer. No British firm could do the job. Finally, after the launch of a £500,000 bond issue in 2004 to provide emergency funding, the order was placed with one of the last remaining manufacturers of steam-engine boilers, at Meiningen in Germany.

But as Tornado pulled into the sleepy halt at Quorn on a last golden day of September, the quarrels and frustrations of its construction were forgotten. Supporters cheered, photographers snapped, crowds applauded and excitement took hold. Who said the old days of main-line steam were gone? The engine still lacked its final coat of apple green, the livery of the old London and North Eastern Railway. But even in its grey undercoat, it shone with freshness.

Enthusiasts and covenanters knew all the engineering details: the efficient Kylchap exhaust system that forces extra work from the steam as it reaches the blastpipe; the 20-ton boiler with a 3,141 sq ft heating surface; miles of copper piping; the 43 superheater flue tubes and the vast coupling rods that transmit extraordinary power from pistons to driving wheels.

There are a few innovations. A full electronic signalling and recording system has had to be tucked into the cab, under the driver's seat, so that Tornado can conform to safety regulations. Every movement of the regulator, every fluctuation in speed, can now be recorded electronically to provide a record of how the engine is driven.

Tornado made four runs that day, from Loughborough to Leicester and back. And in a few weeks one of the mightiest locomotives built in the land where steam engines were invented will be plying the mainline, whistling in triumph.

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