IT may look like a toy, but an artefact just bought for the nation could prove to be the missing link of the engineering world.

Experts are agog with excitement about an object that some believe could help unlock the engineering secrets of the Industrial Revolution.

The National Railway Museum (NRM), in York, has acquired what could be one of the oldest steam locomotive models in the world - an item whose origins are shrouded in mystery.

The working model, known as Sans Pareil, was bought for £92,000, with National Lottery help, from a private collection on the South Coast. It dates from the early 19th Century and was believed to be the work of Timothy Hackworth.

His Sans Pareil is on show at the Locomotion museum in his home town of Shildon, County Durham. It was one of five locomotives that took part in the 1829 Rainhill Trials eventually won by George Stephenson's Rocket.

But research now suggests the NRM's Sans Pareil may be far older - and there is evidence it may have been designed by the grandfather of the railways, Richard Trevithick.

Specialists suspect it is one of Trevithick's earliest working prototypes, when he was experimenting with a double-cylinder boiler for the first time.

If so, it is the forerunner not only of the modern railways, but of virtually all modern engines, including cars and aeroplanes. As such, it could be the technological equivalent of evolution's missing link which, in turn, would make it priceless.

Scientists and historians will now examine the model's design and manufacture in detail, analysing the metal used in its construction and even examining the boiler residues.

The NRM's rail collections manager, Jim Rees, said: "This is a fantastic item to be acquired for the nation.

"This may be one of the first model railway engines in the world - but this model was not built as a toy. It was to test new engineering concepts.

"The Sans Pareil model is very different in design to Hackworth's locomotive of the same name.

"But we know Trevithick used the same name in his own early work. If this is indeed Trevithick's work, it is one of his earliest designs, experimenting with the working relationship between two cylinders. It could be a very exciting discovery indeed."

Industrial archaeologist Dr David Gwynn said: "The double immersed cylinder on Sans Pareil resembles other locomotives by Trevithick and is similar to those used around 1811.

"That particular design concept was never used by Hackworth."

The model will go on display at the NRM from Monday until September, when it will be sent for analysis