Railway Air Services
Railway Air Services was set up in 1933 by the four railway companies and Imperial Airways. They were awarded a Post Office contract to carry mail on their routes with no extra fee charged. They were also able to carry letters, which received the RAS cachet, under the Railway Letter Agreement. This service ceased when the Post Office contract expired on 30th November 1934 and awarded to Hillman Airways, and was not revived when the contract reverted to Railway Air Services from 2nd December 1935.
20th August 1934.
RAS 3d PAID cachet.

Railway Air Services Air Mail Magazine Volume 1, pp. 72-73, A. Phillips (1939)
Official British Inland Airmails Part 3. John C.W. Field, British Philatelic Bulletin, Volume 24 Number 7, March 1987
The Story of the Airway Letter Service 1933-1988 D. Wilson, Air Mail News June 1994
The Airway Letter Service Supplementary Notes D. Wilson, Air Mail News December 1994