Hercule Poirot mentions at the end that he has never traveled on the Orient Express, raising viewer expectations of his most famous case, "Murder on the Orient Express."
The song that plays at the end credits of "Mystery of the Blue Train" is the same that plays in the opening scene of the following episode "Cards on the Table" so that when you are watching by streaming, it is nearly a seamless transition.
Agatha Christie occasionally reworked her short stories into full length novels; sometimes removing, adding, or switching out the lead detective. The Mystery of the Blue Train was preceded by the Poirot short-story that had been adapted in season 3 The Plymouth Express (1991). Character names and details were altered, but the basic plot remains similar.
Lady Tamplin's open-top roadster is a Delahaye 135MS.
Nicholas Farrell (Knighton) was also in season 4 episode The ABC Murders (1992), playing Donald Fraser.