Which cargo tank specification is built as a double-walled, insulated vessel with a vacuum-jacket option, designed specifically to keep cryogenic liquids at the required low temperature during transport?

49 CFR 178.338 (MC 338 cargo tank specification)

Cryogenic liquids like liquid nitrogen or oxygen have to stay extremely cold or they boil off rapidly, so the tank itself has to do the work of an insulated thermos on wheels. A driver or inspector who can recognize an MC 338 tank understands why it looks and handles differently from an ordinary liquid tanker.