TNT minecarts don't explode when pushed off a rail with a piston or if the rail underneath it is destroyed with water. After the rail is removed, the player can not cause it to blow up.

What I expected to happen was...:
The cart would explode when making contact with the ground.

What actually happened was...:
The cart doesn't explode.

Steps to Reproduce:
Piston
1. Place a sticky piston next to a track with a TNT cart on it.
2. Extend and retract the piston causing the rail to retract with it.
3. Cart makes contact with ground and doesn't explode.

Water
1. Place a bucket of water flowing onto the minecart rails
2. Remove water
3. Minecart makes contact with the ground and doesn't explode.