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.

It's the last stage of the tiny train's epic journey. She's in bad shape and still has to face bad weather, rocky terrain and the steepest part of the route before she can reach Inverness.