From this book, it seems you would seem to be able to tell a lot about characters by their nationalities. For example, we learn that the British are reserved and have a problem with their feelings, while Italians are hotheaded and really love knives. We are sometimes encouraged to read against these generalizations, though not always.
For example, M. Bouc suspects the Italian man and declares that "an Italian's weapon is the knife, and he stabs not once but several times" (2.5.76). But Poirot responds that it is "hardly as simple as that" (2.5.80). However, Poirot's assertion that an "Anglo-Saxon brain" plotted the murder is never contested (2.10.55).
This emphasis on stereotypes is one of the more problematic aspects of the book for today's readers.
The train conductor suggests that this is the kind of crime a woman would commit. These assumptions are challenged, as when it is proved that many of the women on the train have incredibly strong wills.
Assumptions are made about characters based on their dress and appearance, much of which has to do with class. For example, the American is derided because he chews gum:
"There is a large American on the train," said M. Bouc, pursuing his idea – "a common-looking man with terrible clothes. He chews the gum which I believe is not done in good circles. You know whom I mean?" (1.5.134)
Also, the first-class passengers are interviewed first. As Poirot says, perhaps a little coyly, "I have become what they call a snob" (2.8.2).