1944 January and February, “Chime Whistles and Horns”, in Railway Magazine, page 2:
For use with petrol-electric, and subsequently diesel-electric, vehicles, air horns were developed about 1918, and today they vary from single-note instruments to those producing as many as five chiming pitches.
1962 March, Brian Haresnape, “Design in 1961—a Retrospect”, in Modern Railways, page 191:
At a later date in production, the Western Region requested that the two-tone air horns should be placed on the cab roof instead of behind the buffer beam, in order to increase audibility.