HazMat Experts and Firefighters petition Dow Chemical and Union Pacific for safe rail tank cars transporting gas chlorine. Secondary containment is a necessary improvement that must be implemented. See–PETITION C KIT for First Responders Comments.
Submarines.
Think of submarines. You seem to be thinking of emergency containment perhaps after the fashion of a coffer dam around an oil tank, or perhaps a containment building over the top of a nuclear reactor.
Here it is the opposite. It is more like a submarine. It is a pressurized containment vessel that contains the gas (or the crew) and it is wrapped inside of a protective outer hull. The outer hull does not hold the pressure, there is no need for it to do so: all it needs to do is to protect the inner hull.
That said, you must still transport hazardous goods, you need to do it as safely as possible, but nothing is fail-safe. Even submarines can be sunk or destroyed by a torpedo, a tank car no matter how well built can still be damaged to the point of leakage, and if that is the case, adding a diaper to it will not help contain the disaster.
ROAR