This was the 6th Poirot book published, so it's a pretty early one. It comes after the extremely unfortunate The Big Four (which is terrible garbage), so I was very curious to see how Christie would right the ship and get Poirot back on track.
Well, she doesn't ... not entirely. This book is certainly better, and much more in the spirit of the better Poirot novels, but it's still decidedly a lesser effort.
The plot has far too much "gangster crap" in it - dramatic action scenes of the worst, most cliched type, but which Christie loves to cram into these books. It also has a lot of stereotypes, which she may have found thrilling and adventurous at the time, but which are sad, thin shadows of the rich and rounded characters she develops in later books.
By "gangster crap," I mean plot elements like: sinister jewel thieves; a tough American millionaire with a gun in his pocket; his stalwart war hero valet; Parisian "apaches" (thugs, in casually racist 1920s parlance); priceless rubies being sold by a Greek Jew with an "impassive yellow face and long beard," etc etc.
It's quite poorly written and, to our modern eyes, completely unbelievable. 100 years later, it reads like a parody of a 1920s spy novel.
Then Christie shifts focus to a better, more familiar setting - the domestic unhappiness of the idle rich. She introduces an unhappy romance - the rich American businessman's daughter married to gain a title, but she now hates her husband and wants to run away with a French dandy; meanwhile, the husband is a dashing playboy who in turn hates his ball-and-chain wife, and takes as his mistress an temperamental ballerina with a taste for money and lots of it.
Finally, Christie introduces a young woman who has inherited an enormous sum of money from the elderly woman she was nursing. Suddenly transformed from hopeless spinster into eligible woman of means, she decides to see the world.
All these people happen to take the same train to the French Riviera... and there's a murder on board, of course. And - naturellement! - Hercule Poirot ALSO is on this train. And then the book begins in earnest - who killed the victim, how, and why?
If there is one thing Agatha Christie loves, it's large ensembles of suspects. That's true here also, but I think most of them are quite flat and unidimensional. The only character who really comes across well is Poirot himself, who is filled with funny little mannerisms (like in the casino at Monte Carlo, where he only bets the minimum sum allowable, and only on the even numbers - so like him) and great little turns of speech.
What happens next, in terms of plot and solution, is messy and long-winded. This is also one of the novels that has a rather unbelievable solution. I am not sure that a properly astute reader, paying full attention, could legitimately have solved this crime... and Christie does that thing where she introduces some evidence during the summing up, which is not very fair.
All in all, this novel is sort of a signpost pointing towards better things ahead, and thankfully leaving behind some of her worst tendencies... while exhibiting others. The single biggest and most frustrating element in Blue Train is that Christie is so focused on the stupid espionage/action elements, which are the least interesting.
I would call this either the best of the worst novels, of at the bottom of the middle ones. I'm going to put in the former camp, because it's very messy - the structure is all over the place - and because the crime isn't very believable. But it has a few nice touches.
