Cal. Pub. Util. Code § 531

Every common carrier subject to the provisions of this part may transport free or at reduced rates contractors and their employees engaged in carrying out contracts with the United States, this State, or any county or municipal government, or other governmental agency in this State, and materials or supplies for use in carrying out such contracts, in each case to the extent only that such free or reduced rate transportation is provided for in the specifications upon which the contract is based and in the contract itself.