Hearts and their uses?

I've been cashing in hearts for trains when I hit 100 hearts. That gives you 100 XP or thereabouts. Then I sell the train for $1,000. I like the creepy clown train so I already have 10 or so that I plan to keep. Everything else is XP and cash! :D

P.S. I had my hearts disappear one night as well. They all came back though.