Historians are trying to find out why the plans, dating back to 1945, which would have seen a second bridge being built downstream of the existing one, were drawn up – and why the idea was dropped.

Network Rail archivist Vicky Stretch told The National: “We have a lot of drawings for the Forth Bridge but very much concentrating on the original construction of the bridge we see today. So this is a real mystery. Why these drawings were created, we just don’t know.”

Dr Miles Oglethorpe, of Historic Environment Scotland, said the plans may have been drawn up to replace a bridge damaged in war or to cope with increase demand for trains.

“They date from 1945, around the time V bombs were landing in London, so there was a possibility they were thinking it might have to be replaced,” he said. “But my theory is simply that they were anticipating more rail travel. The existing bridge was being used heavily and they were thinking ahead.”

The Forth Bridge opened in 1890, and remains one of the greatest cantilever trussed bridges, continuing to carry passengers and freight. Its distinctive industrial aesthetic is the result of a forthright and unadorned display of its structural components.

Anyone who knows anything about plans for a second bridge should go to www.forthbridgeexperience.com/contact-us.

Images courtesy of Network Rail