These are Municipal Solid Waste (MSU) cans or trash containers. These are for transporting HazMat to the lower 48 for disposal. It is common to see these move South loaded and return North empty. Pretty regular service for several years now. Typically though they are on a truck chassis and handled TOFC. Very uncommon to see them handled as COFC.

Now don't get excited that this is "double stack" service. Sure it's two containers high. However, these are not normal height containers and are not as big of a wind sail that standard containers are.

Photographs courtesy of Casey Durand