With up to 400 trains a day running on its 62 miles of track, the channel tunnel is the busiest railway system in the world. And doing things at the double is key to its success. Two countries built it. There are two terminals. And there is duplication at both ends. This programme shows how teams of two work on both sides of the channel to keep tourists and truckers moving. But how will preparations for Brexit affect such a well-oiled system of twinning and duplication, finely tuned over 25 years? How will the freight teams cope when car drivers end up in the wrong lanes? How do French cakes keep English teams in shape for handling the kind of chaos that ensues when trucks collide with trains? And is it true that no one chocks better than a channel tunnel chocker?

This episode also explores why two tunnels are better than one. And, although water and electricity are not a good combination, the film follows the catenary team every step of the way as they fix the overhead cables in the 'wet area' of the tunnel. There is also a driver’s eye view of the giant crossover doors that link one tunnel to another in the biggest undersea cave ever built. And the team meet Maurice, the charismatic tunnel travelling bulldog.