(a) Every common carrier subject to the provisions of this part may transport, free or at reduced rates, as follows:
(1) Persons for the United States, state, county, or municipal governments, or persons or property for charitable or patriotic purposes, or to provide relief in cases of general epidemic, pestilence, or calamity.
(2) Contractors and their employees, material or supplies for use or engaged in carrying out their contracts with such carriers, for construction, operation, or maintenance work or work incidental thereto on the line of the issuing carrier, 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.
(b) Common carriers may also enter into contracts with telegraph and telephone corporations for an exchange of service.