I think I can't

Ben and I were playing with his trains the other day, and he had his Little Engine that Could engine. This engine has a tall smokestack, and it doesn't fit under his tunnels and bridge arches. Ben would have the engine start up the hill to the bridge, and said "I am the engine that could! I think I can, I think I can, I think I can!" Then he'd get to the top of the bridge, where the engine does not fit, and would gleefully say "I can't!"