Graham Greene was a prolific lover, primarily of prostitutes and other men's wives. In the 1940's, Graham had an affair with Dorothy Glover, a costume designer, who later began a career as a children's book illustrator under the name Dorothy Craigie. [She also wrote a few children's books under the name David Craigie. Bear Alley is a great illustrator blog that has the most comprehensive info I could find online about Glover. It's not much.]
Greene's The Little Train was originally published in 1946 as Craigie's work, and was republished as Greene's in 1957, long after they had broken up.
Craigie's art is really wonderful, with bold, flat color that feels very contemporary. Her cover of The Little Steamroller reminds me of Richard McGuire's artwork a bit.
Craigie/Glover sold papers and manuscripts over the years. There's a box of stuff-- including original artwork for The Little Fire Engine--in the Georgetown University Library Special Collections, which I'm already making plans to visit.
The Georgetown page also has a helpful list of other titles by Craigie/Glover, forgotten classics like
Captain Flint Shipwrecked, Parrish, London, 1959
Nicky and Nigger Join the Circus, Parrish, London, 1960
Nicky and Nigger and the Pirate, Parrish, London, 1960