One of the most fascinating aspects of Christie's novels is the period in which they are set: usually the years between the two World Wars. The world is changing rapidly during this time. There are new technologies, advances in science, and world-altering discoveries. Christie's novels give you a glimpse of this period, and new modern conveniences (and inconveniences) are important to the plot. In Murder on the Orient Express, the modern train plays an important role. However, the lack of modern communication tools and contact with the outside as a result of a snowstorm actually helps Poirot solve the murder.
If the Orient Express hadn't stopped moving, no one would have ever discovered the real plot to kill Ratchett.
Though we live in an increasingly modern world, human nature remains the same.