49 C.F.R. § 213.103 : Transportation — OTHER REGULATIONS RELATING TO TRANSPORTATION — FEDERAL RAILROAD ADMINISTRATION, DEPARTMENT OF TRANSPORTATION — TRACK SAFETY STANDARDS — TRACK STRUCTURE — Ballast; general

Unless it is otherwise structurally supported, all track shall be supported by material which will-

(a) Transmit and distribute the load of the track and railroad rolling equipment to the subgrade;
(b) Restrain the track laterally, longitudinally, and vertically under dynamic loads imposed by railroad rolling equipment and thermal stress exerted by the rails;
(c) Provide adequate drainage for the track; and
(d) Maintain proper track crosslevel, surface, and alinement.

49 C.F.R. §213.103