Hercule Poirot was making a comparison on how a train ride had the same dangers of a sea voyage. If we think and see every single detail it does have their similarities. They both are massive transportation machines that are capable of handling a large amount of people within their parameters. That means that for a certain amount of time you are "stuck" with the same people and unable to go anywhere else. In consequence, if something bad happens on the ride then there is going to be a 50/50 chance in everything. If it's the case of being the person who is hurt or being the person who is hurting people. Those are the types of dangers the passengers may have.
