30 CFR § 77.1601
Transportation of persons; restrictions.
No person shall be permitted to ride or be otherwise transported on or in the
following equipment whether loaded or empty:
(a) Dippers, shovels, buckets, forks, and clamshells;
(b) The cargo space of dump trucks or haulage equipment used to
transport coal or other material;
(c) Outside the cabs and beds of mobile equipment;
(d) Chain, belt, or bucket conveyors, except where such conveyors are
specifically designed to transport persons; and
(e) Loaded buckets on aerial tramways.