I believe that everyone did it. There are many reasons why I believe this and throughout this paper I hope that you will understand why.
Just after midnight, a snowdrift stops the Orient Express in its tracks. The luxurious train is surprisingly full for the time of year, but by the morning there is one passenger less. An American lies dead in his compartment, stabbed twelve times, his door locked from the inside. These are some of the facts already given from what I have read so far.
One of the first things that lead me to this conclusion was the number of times that Mr. Ratchett was stabbed, twelve. There are thirteen passengers aboard the train and I know that Mr. Poirot didn't do it. That leaves twelve suspects. Also the book gives the clue that one of the wounds looks as if a left handed person inflicted it, which hints to the conclusion that there was more than one person.
The next clue or evidence leading my attention to the realization that everyone was in on it were the descriptions of the stab wounds. The doctor says "two of them merely scratches and three could be fatal". In my mind that makes at least three or four different people that were all in on it. And in order for that many people to think that they can get away with something, like this murder, and all have good alibis there would need to be a lot of people in on it.
This brings me to my next point. If you are going to help cover up for a murder than you may as well help commit it, right? Because, in my opinion you will get in the same amount of trouble either way.
Now that you have heard...