Oddly enough, I had never noticed the connection between "The Plymouth Express" and "The Mystery of the Blue Train" until the writers of this episode brought my attention to it. Their deliberate use of characters with the same name and other details from the novel allowed me to see the similarity between the stories, despite the fact that the solution to the murder mystery is different in each case (somewhat similar, but still different). So kudos to them for that.
The thing that makes this episode stand out for me is that this is the only episode of the 15 year series (with the possible exception of "Curtain") in which the overall theme is that of unrelenting grief. This is one of the few episodes where the killer is shown cold-heartedly committing the murder, not just firing the gun or poisoning the coffee. The broken hearted father is shown in all his grief. And there is not one light-hearted moment to break up the tension and solemnity. I'm not sure if that makes it any better or worse than others. Quite frankly, the one and only episode so far that I've given a perfect 10/10 to ("The Disappearance of Mr. Davenheim"), I liked for its delightful combination of comedy and clever detection. But I will say that this episode comes the closest to what an actual murder must be like for surviving family members. Not something I would want to watch every time, but an interesting change of pace.
The thing that makes this episode stand out for me is that this is the only episode of the 15 year series (with the possible exception of "Curtain") in which the overall theme is that of unrelenting grief. This is one of the few episodes where the killer is shown cold-heartedly committing the murder, not just firing the gun or poisoning the coffee. The broken hearted father is shown in all his grief. And there is not one light-hearted moment to break up the tension and solemnity. I'm not sure if that makes it any better or worse than others. Quite frankly, the one and only episode so far that I've given a perfect 10/10 to ("The Disappearance of Mr. Davenheim"), I liked for its delightful combination of comedy and clever detection. But I will say that this episode comes the closest to what an actual murder must be like for surviving family members. Not something I would want to watch every time, but an interesting change of pace.