Is the minecart an interactive item, or just background decoration? If it's interactive, then I think outlines outside of it would be fine, as would extra colours for more contrast. If not, then I don't think it's a big problem if it blends in a bit, as long as it's readable as [mine cart].
I like the left one the most as well.
I'd have the interior fade into shadow instead of just having a solid dark bottom, I think that would help the interior read better. I'd also darken the hook/hitch-thing so that it doesn't blend into the rail behind it. You could also scrap it entirely if it's causing problems, or use a different hitching mechanism.
Edit with these changes:

I also stuck a solid dark brown shadow underneath the cart. It's a cheap trick but I think it helps it read better, and gives the cart volume.
(Nitpick: I'd stick the cart a pixel or two higher, as the wheels don't feel like they're on the rails currently. The wheels do hug the rails, but they don't envelop the entire rail xP)