Every day cities around the world rely on the railway to keep them moving. Whether it's commuters travelling to work, tourists seeing the sights or goods going to market, trains take care of them all. This episode takes a closer look at the planet's most incredible metropolitan railways and explores how they help us solve our distinctly urban problems.

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.

Railways encounter a multitude of challenges, but among the most demanding of them all comes when faced with crossing the valleys, waterways and canyons that often stand in the way of their progress. This episode of the brilliant documentary series investigates how the most incredible bridges from around the world help to solve a series of spanning problems. From Scotland's iconic Forth Bridge to a vertigo-inducing viaduct in rural New Zealand and an innovative mechanical lift span in Staten Island New York, they each demonstrate an inspired engineering solution that allow trains to overcome any obstacles that might stand in their way.

Mountains are one the greatest challenges a railway can face. Tough climbs, perilous ascents and extreme weather conditions, have all posed seemingly impossible problems. Daring railway engineering has conquered Earth's greatest ranges, to create some of the most innovative railways on the planet. From tunnelling through the mighty Swiss Alps, clearing the snow bound rails in California's Sierra Nevada, to a 19th Century engineering masterpiece, that conquers a near vertical cliff face in Southern Australia, this episode investigates the solutions that have made these remarkable journeys possible.

Since the dawn of the 19th century, rail networks have overcome almost all nature can throw at them. But there's one environment that's their nemesis: water. Some of the world's greatest Railways have challenged engineering's greatest minds in their efforts to conquer our ‘water worlds'. Whether crossing the cyclone-prone Indian Ocean, travelling under the Bosphorus Strait directly through an earthquake zone, or carved through the sea cliffs of Italian Riviera, each railway has solved a seemingly impossible problem: to create some of the most dramatic train journeys in the world.

For over 150 years, railways have pushed engineering to its limits, in order to open up the world. Trains can now travel further, and climb higher, than ever before. But for train engineers, it's the need for speed that has inspired the greatest innovations of all. Trains can now reach previously inconceivable speeds, and record-breaking railways cross the globe. From mighty powered steam trains, super-sonic rocket sleds and trains that seemingly fly above their guideways; this episode investigates the incredible engineering breakthroughs that have made these trains go faster than ever thought possible.