[3] Medically unfit for military service in World War II, Simmons was appointed a lecturer at Christ Church in 1943, researching imperial history.
His early publications included a 1945 biography of the poet Robert Southey, which led to him being elected a fellow of the Royal Society of Literature.
Apart from running his own department there, Simmons also concerned himself with the college library and with the Senate publications board, precursor of Leicester University Press.
[3] Simmons had been joined at Christ Church in 1934 by Michael Robbins, a school friend from Westminster, who shared his abiding interest in railways.
[3][7] Simmons was active in the foundation in 1975 of the National Railway Museum at York, where a reading room in the library was named after him.
[2] Simmons became an honorary fellow of the National Museum of Science and Industry in 1993, and was awarded an OBE in the 1999 New Year Honours.