I've updated JumboWario's world to make this easier. Here's the file:
https://www.dropbox.com/s/b8yt01cpkcpzycp/Achievements%20Imp...Go up the stairs toward the exit. There's a chest with a minecart. Outside, up the hill, is a long rail. Place the minecart, go to the right. It should pop. If not, turn around (don't get out) and go to the other end.
His original world is in this forum:
Minecraft World for All 1,000 Gamerscore