From the planet's deepest jungles to its sprawling deserts and mightiest mountains, railways have been instrumental in helping us conquer them all. This episode explores the wilderness lines that have pushed the frontiers of human endeavour into previously unknown territories, solving a set of complex problems along the way. Whether it's a vast canyon spanning trestle bridge in the depths of the San Diego desert, a train that travels through a mountain to reach one of Switzerland's most imposing peaks or the incredible story of prisoners of war battling to build a now infamous jungle railway, they all exist as proof that engineers can overcome the very wildest challenges on Earth.