Episode 4

The teams face a series of tricky challenges on one key day as they race to get to Inverness. First the tiny train must cross the 60-metre Aberchalder Viaduct. Dick decides to run an electric loco rather than a steam one, but the train team refuse. They're purists and insist on returning to the steam engine, which they claim they can run faster with a new enhanced fuelling system. After negotiating its way through a packed Fort Augustus, the loco must cross a curving trestle bridge that spans a rocky stream. Meanwhile, the build team struggle to find a way to climb a flight of 12 steps on the approach to Inverness. They settle on a helix - a technically demanding but beautiful solution.