Section, acts June 16, 1933, ch. 101, Sec. 5, 48 Stat. 305; Apr. 24, 1935, ch. 78, 49 Stat. 161; Aug. 29, 1935, ch. 816, 49 Stat. 991, provided for cancellation on or before March 31, 1936, of contracts for transportation entered into prior to June 16, 1933