Who killed Edward Ratchett in the Murder on the Orient Express Book?

In "Murder on the Orient Express" detective Hercule Poirot finds himself on a train in Europe when a murder occurs. Upon investigation, he learns the victim is Edward Ratchett — who was almost convicted for killing Daisy Armstrong, but wasn’t punished due to bribery.

Check out why Ratchett kidnapped and murdered Daisy here

Ratchett's death

The circumstances assure the murderer couldn’t have left the train after his crime and must be in that compartment. After some time, Poirot finds that each of the passengers couldn’t have acted alone — as every one of them was involved with the family of Daisy Armstrong in some way.

What do you think this was based on?!

At the end, Poirot decides to present the suspects and the other investigators with two theories:

Unknown killer

The first theory states that an unknown man snuck into the wagon, killed the victim, and escaped unseen. However, the inspector rules this out as having been impossible — leading to theory number two...

Multiple murderers

The second, and correct theory suggests that all passengers and the wagon conductor worked together to kill Edward Ratchett.