49 CFR 179.13: Tank car capacity and gross weight limitation.

Except as provided in this section, tank cars, built after November 30, 1970, or any existing tank cars that are converted, may not exceed 34,500 gallons (130,597 L) capacity or 263,000 pounds (119,295 kg) gross weight on rail.

(a) For other than tank cars containing poisonous-by-inhalation material, a tank car may be loaded to a gross weight on rail of up to 286,000 pounds (129,727 kg) upon approval by the Associate Administrator for Safety, Federal Railroad Administration (FRA). Tank cars must conform to the conditions of the approval and must be operated only under controlled interchange conditions agreed to by participating railroads.

(b) Tank cars containing poisonous-by-inhalation material meeting the applicable authorized tank car specifications listed in § 173.244(a)(2) or (3) or § 173.314(c) or (d) of this subchapter may have a gross weight on rail of up to 286,000 pounds (129,727 kg). Tank cars containing poisonous-by-inhalation material not meeting the specifications listed in § 173.244(a)(2) or (3) or § 173.314(c) or (d) may be loaded to a gross weight on rail of up to 286,000 pounds (129,727 kg) only upon approval of the Associate Administrator for Safety, Federal Railroad Administration (FRA). Any increase in weight above 263,000 pounds may not be used to increase the quantity of the contents of the tank car.