From the novel Curtain we know that Poirot dyed his hair. Agatha Christie also mentioned his being Catholic and made occasional references to his going to church. Also Poirot tells us little about his background and admits to lying about it at times to put people at ease and get them to talk.
Cast member Jack Farthing, as of 2018, has now appeared alongside two different Poirots: John Malkovich here in this mystery, and David Suchet in The Big Four (2013) (episode 13.2).
This series eschews Poirot's trademark "moustaches" in favor of a goatee beard.
Rupert Grint and Shirley Henderson have worked together before on Harry Potter and the Chamber of Secrets (2002) as Ron Weasley and Moaning Myrtle respectively.
The train conductor wears a "Flash and Circle" button on his lapel. The Flash and circle was the symbol of the British Union of Fascists. When he returns Poirot's punched ticket he purposely drops it on the floor, most likely because he believes Poirot to be French after Poirot thanks him saying "merci".