49 CFR § 215.121 - Defective car body.
A railroad may not place or continue in service a car, if:
(a) Any portion of the car body, truck, or their appurtenances (except wheels) has less than a 2 1/2 inch clearance from the top of rail;
(b) The car center sill is:
(2)Cracked more than 6 inches; or
(3) Permanently bent or buckled more than 2 1/2 inches in any six foot length;
(c) The car has a coupler carrier that is:
(3) Non-resilient and the coupler has a type F head.
(d) After December 1, 1983, the car is a box car and its side doors are not equipped with operative hangers, or the equivalent, to prevent the doors from becoming disengaged.
(e) The car has a center plate:
(1) That is not properly secured;
(2) Any portion of which is missing; or
(3) That is broken; or
(4) That has two or more cracks through its cross section (thickness) at the edge of the plate that extend to the portion of the plate that is obstructed from view while the truck is in place; or
(f) The car has a broken sidesill, crossbearer, or body bolster.