Special edition of the engineering challenge, celebrating the 200th anniversary of Richard Trevithick's invention of the steam locomotive. Three teams have 20 hours and the materials they manage to scavenge from the scrapheap to build steam, diesel and electric-powered versions capable of pulling a 20-tonne passenger carriage, before putting their creations to the test in a face-off between the golden age of rail travel and its modern-day equivalent.