About 145 years ago, the 700-ton barque Thomas set sail from the Clyde bound for Halifax, Nova Scotia, laden with locomotives and parts for the fledgling Canadian railways.

She never made it.

Two days into the voyage in 1857 she was fog-bound and becalmed off the Hebrides and then driven on to the rocks of Islay by the unforgiving Atlantic currents.

Her crew were all saved but the cargo, then worth a fortune of about £16,000, went to the bottom, where it was thought to have been lost forever.

But the sea has a way of giving up its secrets, and after one of the longest projects in diving history, experts are getting a priceless insight into early railway history.

Now the entire fascinating story is being told in an exhibition, Treasures from the Deep, at the National Railway Museum in York.

Salvage began in 1977, when a team of RAF divers began to investigate rumours of a lost cargo ship and finally pinpointed the site.

"It's very difficult to dig artefacts out of the ocean bed when you are being thrown against the rocks," said now-retired team leader, Squadron Leader Mike Cameron.

The team were joined by civilian divers, including railway historian Dr Michael Bailey, and continued over a period of several years.

The last dive in Operation Iron Horse, as it came to be known, was in 2000, and over the years a wealth of artefacts were brought to the surface, including some big surprises.

Treasures from the Deep continues at the National Railway Museum until January next year.