Agatha Christie occasionally reworked her short stories into full length novels; sometimes removing, adding, or switching out the lead detective. In the case of The Plymouth Express, it was expanded into the novel that was adapted in season 10, The Mystery of the Blue Train (2005), Poirot remained but other character names and details were altered.
One of the issues of The Evening News read by Poirot and Hastings features the headline "Sir Malcolm Goes at 304 M.P.H." This refers to Sir Malcolm Campbell setting the land speed record in the Campbell-Railton Blue Bird at the Bonneville Salt Flats in northwestern Utah on September 3, 1935. Campbell was the first person to travel at more than 300 miles per hour in a vehicle on land.
While on the train, Florence Carrington is reading Radio Pictorial #34 (September 7, 1934).