49 CFR 230.108: Steam locomotive leading and trailing trucks.

(a) Maintenance. Trucks shall be maintained in safe and suitable condition for service. Center plates shall fit properly, and the male center plate shall extend into the female center plate not less than 3/4 inch. All centering devices shall be properly maintained and shall not permit lost motion in excess of 1/2 inch.

(b) Safety chain required. A suitable safety chain shall be provided at each front corner of all four wheel engine trucks.

(c) Clearance required. All parts of trucks shall have sufficient clearance to prevent them from interfering with any other part of the steam locomotive.