49 C.F.R. § 213.139 – Spring rail frogs.

(a) The outer edge of a wheel tread shall not contact the gage side of a spring wing rail.
(b) The toe of each wing rail shall be solidly tamped and fully and tightly bolted.
(c) Each frog with a bolt hole defect or head-web separation shall be replaced.
(d) Each spring shall have compression sufficient to hold the wing rail against the point rail.
(e) The clearance between the holddown housing and the horn shall not be more than one-fourth of an inch.