Without having read 'Last Train to Istanbul A Novel', we can only speculate. There could be a central character, perhaps a young man who has lost his family in a war and is now on this last train to Istanbul in search of a new beginning. Then there might be an older woman who has wisdom to share and acts as a sort of mentor figure to the younger characters. There could also be a group of friends who have different reasons for being on the train but are united in their shared experience, and their relationships and how they develop during the journey would be an important part of the story.
Since I haven't read the novel, it's hard to say for sure. However, it's likely that there are characters who represent different aspects of the society at that time. There might be a brave and determined protagonist, perhaps a journalist who is documenting the journey and the stories of the people on the train. There could also be an antagonist, like a corrupt official who tries to disrupt the journey for his own gain. And of course, there would be other supporting characters such as the train conductor, a mother with her children, etc.