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.