Believe it or not, the Simplon Orient Express was – and still is – an actual train line that Agatha Christie herself rode.
Why does Christie use this train as the setting of her story? Well, we've come up with a few reasons why it fits.
First of all, the train passes through several different countries. That makes it an international train – and the passengers are entirely international, too.
Second, the train is an invention of the modern age. This makes it a perfect place to dramatize ideas about modern justice (which is exactly what the book does).
Third, it's a small, confined place where everyone is stuck together. That means that the murderers, the victim, and the investigators are all thrown together. No one can really skip town, which makes for a fun and suspenseful read.
Finally, the train itself is split up into first and second classes. The separation makes us keenly aware of the passengers' social standing and place in the world – key factors to solving the mystery.