An exploration of Europe's longest rail network - which still uses steam locomotives. How have these wonderful but out-dated machines remained part of one of the world's most sophisticated railway network?

It is surrounded by the stunning beauty of the German Harz Mountains, but the railway has a murky, hidden past. It was used to transport thousands of Russian and Jewish prisoners from the Mittelbau Dora concentration camp during World War Two. They were used as slaves to build the weapon that Hitler believed would win the war for Germany – the V2 Flying Bomb. The Harz Railway was also used in Operation Paperclip – the secret American programme that saw 1600 German scientists, engineers and technicians spirited away to the Allied side in 1945.

And that's not all. Today's tourists and commuters might not realise as they travel the line that only a few decades ago East German soldiers would routinely board trains to look for dissidents trying to escape the oppression of the communist regime.