55. The power to regulate commerce among the several States extends to commercial highways and to agencies employed in such commerce. Thus waterways capable of navigation and the free and unobstructed use of them are subjects of congressional legislation under the commerce clause. From this it follows that Congress legislates concerning these waterways, their protection, their dredging, the bridges that cross them, the boats that navigate them, the form, size, construction, command, and equipment of these boats, the inspection of boilers, the licensing of officers,—indeed, concerning navigation in its broadest application under the commerce clause. Vessels engaged in such commerce are described as “the public property of the nation, and subject to all the requisite legislation of Congress.”[140] 56. In like manner, the national power to regulate commerce extends over interstate commerce when carried on by land transportation. Thus cars on railroads used in interstate commerce must be equipped with automatic couplers and continuous brakes, and locomotives with driving-wheel brakes.[141] To what length this regulation of commerce may be carried by Congress is unknown, nor can it be determined in advance. The limitations, if any, are of expediency.[142] Thus in exercise of this vast power Congress may regulate hours of labor, wages, selection and use of material in construction of vehicles engaged in such commerce; the education, training, and conduct of persons engaged in handling such commerce; the age of employment; and physical equipment for the welfare of employees, as well as tariff rates and other incidents.[143]