The Biggest Little Railway in the World

Dick briefs the nervous but excited volunteers and selects four team leaders - debt collector Lawrence Robbins, science fiction writer Jenny Kirk, school caretaker Charlie Fox-Wilson and apprentice project manager Paul Burkitt-Gray. He also selects two teams who will build especially challenging constructions such as bridges and viaducts. The volunteers get stuck in straight away.

As the little train starts the first part of its epic journey, it becomes clear exactly how difficult the challenge will be. The team's attempt to use the miniature train ferry that they've built doesn't go well, with the train drifting towards a wall of water... Meanwhile, the track teams are battling in rainy and midge-infested conditions to lay more track. And the build team have to work out how to span a 60-metre viaduct by Aberchalder Bridge. It could flood at any point, so they decide to build another viaduct, but the sheer length of it makes it very challenging. And the train is making slow progress, with much more rugged terrain still to come.

On its first gentle hill, the tiny train grinds to a halt. Bearing in mind the monster hills that it must still face, this is alarming. But one of the train team has a brainwave... Further ahead near Fort Augustus, Claire has come across an impassable rocky stream. After a conference with her team, they decide to build an ambitious curving trestle bridge. But the train doesn't look like it will make it that far. It keeps falling off the track, so Dick drives into the nearest town to see if he can manufacture a bespoke tool that will stabilise the track. After six days of non-stop building everyone's getting tired so they reward themselves with an exuberant birthday party for one of the team members, led by Lawrence the DJ.

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.