According to local folklore, the train vanished near Ksiaz castle, two miles from the south-eastern city of Walbrzych.
Fortune-hunters have looked for it for decades, and in the communist era, the Polish army even carried out several fruitless searches.
This month, however, two men, a Pole and a German, said they had found a train with guns and valuables.
The two men said through lawyers that they wanted 10% of the value of anything that was found as a reward.
So far no visual evidence has been offered of the train's existence.
However, a leading Polish cultural official has now dropped the strongest hint yet that there is truth to the men's claims.
At a news conference, Deputy Culture Minister Piotr Zuchowski said: "In connection with the published information referring to the find of the so-called 'gold train' in the region of Walbrzych, an increase in the activity of treasure hunters has been observed.
"I am appealing to people to stop any such searches until the end of official procedures leading to the securing of the find."
Mr Zuchowski warned treasure hunters to stop looking for the train because it could be mined and dangerous.
"Inside the hidden train - of whose existence I am convinced - there could be dangerous materials from the time of World War II," he said.
"There is a great chance that the train is mined."
Walbrzych's deputy mayor told journalists that the train's location was being kept under wraps, as well as identity of the two men.
"The find is within our administrative boundaries," said Zygmunt Nowaczyk.
"I cannot of course reveal the exact place."
Author Tom Bower told Sky News it could be an art train as there were many of them that crossed occupied Europe and contained huge art collections as the Nazis plundered galleries.
He said: "If it is an art train there will be a huge amount of paintings, perhaps diamonds, rubies, precious stones."
He pointed out the "amber room" has never been found and could be in the train. It was a chamber decorated in amber panels backed with gold leafs and mirrors in a palace outside St Petersburg.