Runaway Train Fireman Sam : Season 8 Episode 2

Nipper sniffs temptation when Bronwyn gives Gareth a sausage sandwich for lunch and snatches it when Gareth leaves it dangling in a bag on the train's brake, but the naughty dog also dislodges the brake, causing The Pontypandy Flyer to head off across the mountains with a carriage full of people and just Nipper in the cab.