I managed to make a circle around my fortress walls, and set some marksdwarves going round during a fairly large elven siege (I got fed up with trading about three DF years ago and now everyone hates me because I killed their merchants and stole the goods (except the outpost liason)). I'm not sure what happened to route, it may have been broken/stolen, but the last I saw of my highly skilled marksdwarves was them flying off the fortress into a crowd of elves. The carts did some serious injuries, but so did the elves.
Message from this encounter: Do not let dwarves fly off castles. They will die. You may however use minecarts to decapitate elves.
Yeah, that's about it for minecarts in my fortress. I used them to drop some magma into my torture chamber, but the goblin within it has gone berserk and I can't get in there to clear the magma.