Resolution: Works As Intended
Windows Vista, Java ver: 1.7.0_51
The hitbox of TNT minecarts is too large to stack them on a single rail, the hitbox of the rail is overshadowed by the Minecart hitbox, making single rail stacking difficult.
1. Place rail down.
2. Place TNT minecart down on rail.
3. Attempt to place second TNT minecart on same rail.
4. Cannot reach rail hitbox without moving minecart.