"Because Princess Celestia's faithful student has to get to work somehow."
Experimentation with lighting angles and shading. Death to my homework.
Edit (1/19/12): I realize Twilight's horn is missing; that's actually an oversight on my part when I was looking at vectors. Whoever drew it didn't trace her horn, so I'm sticking with the "Discord stole it" cover story.
I don't own Bay Area Rapid Transit, or My Little Pony: Friendship is Magic.