S7·E25 Trusty Rusty

Rusty is pulling a train across the old wooden bridge when he notices that some of the joints are loose. He goes and tells the Fat Controller who closes the bridge until it is mended. Rusty then tells the other engines, but Duncan thinks Rusty is just being a silly little diesel and puffs away without refilling with coal. Later on Duncan needs coal, and the only coal available is across the bridge, so off he goes. But Duncan runs out of steam halfway across and can't move, and then the bridge starts collapsing...