In And Then There Were None Everyone's fate was sealed right from the get go. So no matter what happened they were doomed from the start. But in Murder On Orient Express everybody has a chance to live. They aren't doomed right from the get go. In And Then There Were None everybody is from England So they all have the same nationality. But in Murder on Orient Express all the people are from different places. The different setting probably has something to do with that because on a train most of the time its people going all over the place. On a And There Were None all of these people were either going there to work or for vacation purposes.
There are many ways a writer can have books that are in common sometimes it's just the way they start the book. Other times they can have differences like everybody's fate right from the beginning. Agatha Christies from what i read knows how to write her books so you can link them together in some ways and also knows how to make them