No tax shall be imposed by section
4051 on any of the following articles:
(6)
Trash containers, etc.
Any box, container, receptacle, bin or other similar article—
(A)
which is designed to be used as a trash container and is not designed for the transportation of freight other than trash, and
(B)
which is not designed to be permanently mounted on or permanently affixed to an automobile truck chassis or body.
(7)
Rail trailers and rail vans
Any chassis or body of a trailer or semitrailer which is designed for use both as a highway vehicle and a railroad car. For purposes of the preceding sentence, piggy-back trailer or semitrailer shall not be treated as designed for use as a railroad car.