What do you think Poirot means when he notes, "He is susceptible, our Colonel"? Why would he say that the train is "as dangerous as a sea voyage"? What type of danger is he referring to?

An inference that can be made is that the princess is bothered about the tiger door. although there are reasons for both this one has a better reason. the princess is not a monster and even if she didn't want to marry this man she wouldn't want him to die. the grudge of death is pretty heavy. unless she has done this before and feels no regret then she wouldn't want to be married to someone of low quality, if he is low quality.