Did Stephenson deserve the title Father of the Railways?

The controversial letter was sent from Stephenson to Timothy Hackworth, fellow pioneer of the world's first passenger train from Darlington to Stockton.

It has been donated by Hackworth's great-great-grandchildren to the National Railway Museum, based in York and Shildon, in County Durham.

The letter's re-emergence rekindles one of the longest arguments in railway history.

The letter confirms that Stephenson was still using inefficient bellows to help power his engines more than six months after Hackworth introduced the breakthrough blast pipe technology that paved the way for reliable steam-hauled trains.

Built in 1827, Hackworth's The Royal George locomotive featured a blast pipe and succeeded in saving the Stockton and Darlington Railway £532, compared with the cost of horse-drawn trains.

But it was Stephenson who took credit for the blast pipe breakthrough after his revolutionary locomotive, Rocket, built by his son Robert, won the Rainhill Trials of 1829 and hauled the first inter-city from Liverpool to Manchester the following year.

Colin Divall, professor of railway studies at the University of York, said: "Fans of Stephenson have always argued that the replacement of the bellows with the blast pipe, which was a distinguishing feature of Rocket, was George's idea, but this letter seems to cast some doubt on that.

"There were only three key changes in the design of early locomotives that made subsequent engines more efficient than horse power, one of which was the blast pipe.

"And, of course, it was this breakthrough in efficiency that set the railway revolution in motion, and helped earn George Stephenson the nickname Father of the Railways.

"However, the blast pipe is almost certainly an instance of the same device being invented by two people at more or less the same time."

Hackworth's descendents have long extolled the ingenuity of their ancestor, and believe the letter proves that he is indeed the 'unsung hero' of yesteryear.

Jane Hackworth-Young, great-great-granddaughter of Timothy Hackworth, who has been the main instigator of the transfer of the collection which includes the letter to the NRM, said: "We believe people simply assumed at the time that Stephenson had invented the blast pipe.

"That assumption continued through the years, and George Stephenson was happy to take the credit.

"Gradually though, thanks to developments such as Locomotion: The National Railway Museum at Shildon, both historians and the general public are beginning to recognize my great-great-grandfather's contribution to railway history."

Jane added: "I remember my father used to keep several large trunks in the loft of my childhood home, and whenever I came across them he would always try to explain to me how significant the contents were.

"He was an engineer himself and took very seriously my grandfather's family wishes that the family's railway treasures would one day be accessible to the public within a museum environment.

"So we're delighted that the National Railway Museum will at last receive this important historical collection."

It is hoped that the treasured letter, along with the rest of the NRM's archive collections, will become part of a proposed new £3m project known as Search Engine.

If the museum is successful in its bid for backing by the Heritage Lottery Fund, the controversial document will be fully accessible to visitors for the first time.

Helen Ashby, head of knowledge and collections at the NRM, said: "It's important that people continue to engage in this and other historical debates, and this is why we feel so strongly that the nation's hidden archives should be made available for the public to access on a daily basis."

The blast pipe is positioned just below the chimney in the smokebox on a locomotive.

After it has been used in the cylinder to power the wheels, spent steam is released through the blast pipe and escapes into the atmosphere through the chimney with a characteristic 'chuff-chuff' sound.

As the steam rushes from the blast pipe, it takes some air in the smoke box with it, thus creating a partial vacuum, which draws fumes and hot gases through the boiler from the firebox. This makes the fire hotter and creates more steam, thereby increasing the power of the locomotive.

Timothy Hackworth was born in 1786 and took a large role in the creation of the locomotives Puffing Billy, Locomotion, The Royal George and Sans Pareil, for which he is most famous. He died in 1850.

In addition, the Hackworth collection includes hundreds of letters between his children which throw light on the social history of the times and also the passport that enabled Hackworth's son, John Wesley Hackworth, to leave Russia after he had delivered his father's engine - the first in Russia - to the Tsar