47 CFR § 15.211 - Tunnel radio systems.
An intentional radiator utilized as part of a tunnel radio system may operate on any frequency provided it meets all of the following conditions:
(a) Operation of a tunnel radio system (intentional radiator and all connecting wires) shall be contained solely within a tunnel, mine or other structure that provides attenuation to the radiated signal due to the presence of naturally surrounding earth and/or water.
(b) Any intentional or unintentional radiator external to the tunnel, mine or other structure, as described in paragraph (a) of this section, shall be subject to the other applicable regulations contained within this part.
(c) The total electromagnetic field from a tunnel radio system on any frequency or frequencies appearing outside of the tunnel, mine or other structure described in paragraph (a) of this section, shall not exceed the limits shown in § 15.209 when measured at the specified distance from the surrounding structure, including openings. Particular attention shall be paid to the emissions from any opening in the structure to the outside environment. When measurements are made from the openings, the distances shown in § 15.209 refer to the distance from the plane of reference which fits the entire perimeter of each above ground opening.
(d) The conducted limits in § 15.207 apply to the radiofrequency voltage on the public utility power lines outside of the tunnel.