Section, acts June 29, 1937, ch. 404, title I, 50 Stat. 421; June 16, 1938, ch. 464, title I, 52 Stat. 736; June 30, 1939, ch. 253, title I, 53 Stat. 965, made an earlier specific appropriation available for maintenance and operation of motor-propelled passenger-carrying vehicles.