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.