Embankment to be searched with thermal imaging cameras to verify existence of possible cargo of stolen gold and artworks

Engineers are set to start surveying a railway embankment in south-western Poland to establish how to dig out a “gold train” that is thought to have been buried there in the dying days of the Third Reich.

The existence of a Nazi gold train, its whereabouts and its cargo – possibly stolen valuables and artworks – remain one of the great unsolved mysteries of the second world war.

“In the past 70 years, three cold war secret services – the United States, the Russian, then the Polish – carried out searches,” said Piotr Koper, a 44-year-old builder who claims to have found the suspected armoured train with a fellow treasure hunter. “We succeeded because we are local people.”

Some historians believe up to three trains laden with arms, art, gold and archives vanished in a 18 sq mile area near the present Czech border as the Red Army advanced in 1945. The strategic area includes Hitler’s command post at the grandiose Ksiaz Castle (formerly known as Fürstenstein) and Project Riese, a suspected secret weapons programme.

Project Riese was a network of underground tunnels and chambers dug out beneath the Owl Mountains by an estimated 30,000 prisoners of war and concentration camp prisoners. The earth embankment that will be surveyed this week by teams including one from Krakow’s mining academy rises up alongside the existing Wroclaw-Walbrzych railway line.

Facebook Twitter Pinterest The underground galleries of Nazi Germany‘s ‘Project Riese’ construction project under the Ksiaz castle in Walbrzych, Poland. Photograph: Janek Skarzynski/AFP/Getty Images

It will be checked with magnetic field detectors, thermal imaging cameras and radars. The site, on the outskirts of Walbrzych, has been under police guard since August when Koper and his friend, German-born Andreas Richter, showed the authorities images they had taken with a £7,000 ground-penetrating radar kit.

“Four years ago, we were given information by a witness who was in Walbrzych at the time the train disappeared in April 1945. Radar technology has become affordable so we were able to check the information,” said Koper, who would not reveal if he knew anything about the hiding places of the other two supposed trains.

“The Nazis dug out the embankment, created a junction and laid track to divert the train off to the side. Then they parked the train, which is 90 metres long, removed the rails and put back the soil.”

Since August, the Polish military has cleared vegetation from an area the size of a football pitch. Soldiers have swept for mines and analysed the ground for the presence of poison gas. During the holocaust, Zyklon B – for use in gas chambers – is believed to have been transported on the line.

The treasure hunters’ images show only the outline of what appears to be a train. “We do not know what is inside, only that it is armoured, which suggests a precious cargo,” said Koper. “There could be gold but that is not what interests us. In fact, we were looking for a tunnel when we found the train.”

The men have hired a lawyer and applied to the Polish treasury for a reward of 10% of the eventual value of the train and its contents. “We have worked for four years to get to the bottom of legends that have flown around our city for 70 years. It has become a fascination,” said Koper, who added that he and Richter were well out of pocket for having self-funded all their research.

But others in Walbrzych, a depressed coal mining town with smog-stained blocks of flats and 20% unemployment, are already cashing in. Initial reports of the discovery, in August, brought a stream of tourists bearing metal detectors.

Walbrzych now has a Gold Train Car Wash, a Gold Train Skoda dealership and a line in souvenirs including fridge magnets, stickers, bags and gold ingot paperweights. At the city’s museum, there is a waiting list for gold train mugs.

“I’m no Indiana Jones,” said the district governor, Jacek Cichura, “but my colleagues in the rest of Poland now call me the gold governor. We are in a special economic zone. Life is tough. The young people are leaving to work abroad. But the gold train has brought a tourism boom.”

The influx of visitors is being felt across the district - from the towering Ksiaz Castle to the mist-clad Owl Mountains 10 miles (15km) to the south. According to local legend, the Führer’s castle lair is linked by at least one secret tunnel to Walbrzych and onwards to Project Riese’s underground network. Riese means giant in German, and this was where Hitler’s Wunderwaffe (magic weapons) were supposedly under development.

It is unclear to what extent the underground mega city and arms factory actually functioned and how much of what we know is simply surviving propaganda. But for the past 12 years, carpenter Krzysztof Szpakowski, 56, has dowsed his way to uncovering thousands of metres of passages dynamited into the pine-clad Owl Mountains. Some tunnels are flooded but in others he has found German machine guns and prisoner-of-war graffiti.

An eclectic mix of visitors – from militaria maniacs to bat fanciers – sign up for his guided tours. But Szpakowski himself has the exploration bug. He broke his arm last week when he slipped in one of the Riese’s dank, eerie tunnels.

“This was a closed military area – first Russian, then Polish – until 1991. The soldiers were looking for something. When I was a teenager we knew something had gone on here. But there was no one to ask because all the [German] residents were shipped out of the area and replaced by Poles after the Potsdam agreement.”

Szpakowski, Koper and Richter collaborate in their treasure-hunting by lending each other equipment. But they have different views of what is on board the “gold train”. Szpakowski believes it contains top-secret strategic equipment.

He said: “The Nazis wanted to develop an atomic bomb. They were working on anti-gravity propulsion. There is something on that train that was intended for Riese. We only know 5% of what went on in this part of the world,” he said.