I was making a rig for the TNT carts where a piston replaces the powered rails with the activator rails. I put 4 carts in and then they randomly exploded and then my game crashed. I then tried to recreate the bug, but i couldn't. However when i put the cart on a three long power rail line that was powered from the middle underneath by a red stone torch, the TNT cart exploded without a countdown animation. You can see a video here: http://youtu.be/XuPKDg_ALX8
0%, The video is still uploading so it might not be ready.