How much speed is lost, if any, when a minecart hits an object?
Is it possible to build a cart directly on top of rollers, thus allowing one to start it remotely by linking it to a lever-operated gear assembly?
Is it possible to immerse a magma-safe cart in magma, on magma-safe rollers, linked to a magma-safe gear assembly, thus allowing one to fling a superheated iron box at invaders at superluminal speeds with the flick of a switch?
Minecart danger rooms -- what is the minimum speed that a cart is considered a "dangerous" projectile? By carefully controlling the velocity of the carts, it may be possible to get them to move fast enough for dwarves to dodge them (thus presumably training their Dodge skill) while still not hitting hard enough to cause more than minor injuries.
EDIT: Dwarven airmail -- assuming minecarts are indestructible, it should be possible to rev a minecart up a spiral ramp to the surface, then fling it off a jump into a wall above a pit some distance away, after which it drops down into the fortress itself. Pretty much a Stupid Dwarf Trick at best, but there may be some instances where you need to deliver supplies without having a ground path between the two points.