Find an object in Murder On The Orient Express that is significant to the main character(s) in the story. Explain clearly why the object you have chosen is of importance to the main character(s) in the story, and draw on your own personal experience if necessary.

This is a fairly open question, which means that you can interpet it in several ways.
What first comes to mind for me is that the question is asking about the personal items identified by Poirot in the story, which serve as clues for him and which help him to solve to the mystery—for example, the embroidered handkerchief which he finds in the murdered man's carriage. The handkerchief is significant to Princess Dragomiroff not only because it belongs to her, but also because it reveals several things about her:
1. In the first place, the behavior of her maid when asked about the handkerchief reveals that the maid is extremely devoted to her mistress, so the handkerchief helps to give some illustration about the relationship between the two women.
2. The fact that the letter on the handkerchief is a "H" rather than a "N," allows her to explain her Russian background and the fact that the embroidered letter is Russian for "N," as in Natalia, her first name. The handkerchief tells us about her first name and background and indicates to the reader how foreign she is in the environment of the story.
3. In explaining the handkerchief, the Princess is also able to demonstrate her internal composure and strength of organization to Poirot. She offers to prove her ownership of the handkerchief by contacting the Parisian manufacturers, showing us that she is wealthy enough to spend time in Paris and make purchases there. Poirot is impressed by her "will."
You could answer this question by taking another of the clues Poirot spots—the pipe or the kimono—and explaining these in connection to one of the main characters. Alternatively, you could interpret the question in a different way and consider a significant object that does not directly relate to the murder. You could make some very interesting observations about Poirot himself, for example, based on his meticulous grooming and what this tells us about his personality. He possesses an enormous pocket watch, and he wears patent leather shoes. Poirot is "exotic" and arguably ridiculous—how does this help him in his career?