49 CFR 229.67: Trucks.

(a) The male center plate shall extend into the female center plate at least 3/4 inch. On trucks constructed to transmit tractive effort through the center plate or center pin, the male center plate shall extend into the female center plate at least 1 1/2 inches. Maximum lost motion in a center plate assemblage may not exceed 1/2 inch.

(b) Each locomotive shall have a device or securing arrangement to prevent the truck and locomotive body from separating in case of derailment.

(c) A truck may not have a loose tie bar or a cracked or broken center casting, motor suspension lug, equalizer, hanger, gib or pin. A truck frame may not be broken or have a crack in a stress area that may affect its structural integrity.