Dwarven MIRV. Because this could continue quite ridiculously.
Only if we can figure out how to give the dumped carts some momentum *and* send them all to different destinations (each with a dumping stop)... even though they all start from the same place at the same time. Seems unlikely, which is unfortunate.
This newly-discovered exploit and its
logical continuation will help you solve this problem.
UPD: Allow me to elaborate. There are two ideas behind my concept of Dwarven MIRV:
1) When a cart slams into a wall at a sufficient speed, its contents shotgun 1 Z-level higher, over a comparatively wide surface on which anything can be built. A similar process goes for the fortifications, too.
2) The abovementioned exploit allows the players to create contraptions that will catch flying or derailed minecarts out of the air, put them on tracks and accelerate them to previously inconceivable speeds.
If we set up the minecart-that-contains-minecarts to shotgun its contents, namely minecarts, into the exploit-based accelerators, they will
catch the minecarts, accelerate them to shotgun-worthy speeds and carry them away to the targets of our choosing. Therefore, the Dwarven MIRV, complete with aiming and whatnot, becomes easier to create through the use of accelerators and shotguns rather than track stops.
UPD2: False alarm: a proof-of-concept device has shown a problem of the most unexpected sort, and said problem probably makes MIRV shotguns impossible even in theory.
When nested minecarts shotgun out of a single minecart, they
collide with each other on the first tick after being released. Guess what rock-filled minecarts on supercritical velocities do when they collide with each other? Exactly: they shower everything in front of them with rocks. The premature shotgunation that ensues is entertaining and deadly, but doesn't exactly satisfy my standards for a MIRV warhead.