Bernadette: What is it about trains that you like so much?
Sheldon: What an interesting question. When I was a child, life was confusing and chaotic for me, and trains represented order. I could line them up, categorize them, control them. I guess you could say that they gave me a sense of calm in a world that didn't.
Bernadette: That's lovely, Sheldon.
Sheldon: Well, other than when they're chugging through your bowels, these things are magic.