Spoiler warning: I've been careful not to spoil the solution, but some of the plot points do end up being revealed during this post.
This was apparently not one of Agatha's own favorites, and I can sort of understand why. It doesn't quite come alive in the same way that her very best books come alive. In addition, I feel like her characterization was just a tiny bit off in this one - some of characters were just a bit too much, so they felt more like caricatures than characters in several cases, especially Viscountess Tamplin, Mirelle and the Mr. Papapolous (no idea how to spell his name). In addition, as likeable as Katherine Grey was, men falling over like bowling pins for her was annoying.
I agree with BrokenTune that this feels like a less successful first attempt at Murder on the Orient Express, which was a much better book in every way. Having said that, though, from my perspective, it's still well worth reading.
The relationship between Poirot and Katherine is really darling, and marks the first time that he collaborates with a young woman. This type of collaboration is reprised several times over the series with different characters, and those relationships are some of my favorites. Thank you, Agatha, for not making Hercule Poirot an old lech. And, can I just say how delightful it is to have an older (although how old Poirot is in this book is not extremely clear) male character who enjoys the company of young women in a really fresh, charming, and completely non-creepy way. Even when he admires their looks, it doesn't feel threatening at all. He also makes use of her observations in a way that demonstrates his respect for her perception.
There are a lot of other authors who could take some suggestions from this method.
The mystery isn't as good as some, and poor Ruth really did not deserve her fate. In addition, her murderer is one of Christie's least redeemable characters (which is the opposite side of the same coin from Murder on the Orient Express, where it was the victim who was irredeemable and the murderers who were redeemed). In fact, if Ruth Kettering had been a child, and if the murderer had escaped justice, Mystery on the Blue Train might have been used as the motive for Murder on the Orient Express.
Poirot says about the killer:
"This is not the first murder that lies to [omitted] charge. [omitted] is a killer by instinct; [omitted] believes, too, in leaving no evidence behind... Dead men and women tell no tales."
The murderer in The Mystery of the Blue Train doesn't just murder for gain. He murders because he is a murderer. There was no real reason to kill Ruth Kettering - the rubies could have been stolen without killing her.
And poor Lenox, whom we leave at the end of the book suffering from unrequited love:
"...Life is like a train, Mademoiselle. It goes on. And it is a good thing that that is so."
"Because the train gets to its journey's end at last, and there is a proverb about that in your language, Mademoiselle."
"Journeys end in Lovers meeting." Lenox laughed. "That is not going to be true for me."
"Yes - yes, it is true. You are young, younger than you yourself know. Trust the train, Mademoiselle, for it is le bon Dieu who drives it."
The whistle of the engine came again.
"Trust the train, Mademoiselle," murmured Poirot again. "And trust Hercule Poirot - He knows."
And let's not even talk about Mirelle, who not only ends up prospering after her abominable behavior with respect to Derek Kettering, but who ends up, in a round-about way, as the possessor of the Heart of Fire. Ugh.
I'd really put this in the lower middle tier of Christie's mysteries - she's still finding her footing a bit here. Overall, though, it isn't one of her just for completists books, nor would it be one that I recommend as a starting point.
Thanks for the buddy read, BrokenTune & Lillelara! It was fun seeing all of your updates!