If a TNT cart blows up another one, the blown up TNT cart drops a normal minecart.

Pic of setup: https://www.dropbox.com/s/u7w2ni3nfpls35c/2013-02-15_00.11.07.png
Just place 3 carts on the diagonal rail, with some time inbetween, and wait.
2 Carts will get blown up by another cart, and they will drop normal minecarts.