S6·E18 Percy's Chocolate Crunch

There is a water shortage on the Island of Sodor and Percy is upset because he can't be washed down as often as he would like. He gets very dirty very quickly and so decides to deliver some sugar to the Chocolate Factory - after all, he can't get dirty doing that, can he? But an oil spillage on the track leading straight to the Chocolate Factory spells disaster - and dirtiness!