A boy + chocolate + train

I remember seeing an old movie in the early 2000s with my Mom on TV. In my memory, I guess it was in sepia? I’m sorry, I’m really not sure about that.. The movie had a little boy who was eating a chocolate bar on a train, when all of a sudden he turned into a monster. I remember my Mom’s comment that time was. “See? that’s what happens when you eat too much chocolate”. It’s been bothering me and Googling wasn’t of any use. And no, it’s not “species”.

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