I think it's actually 2.5 stars. I feel bad because others are rating it so highly but the reality is that I was a little disappointed.
I really enjoyed the set up for the plot and the setting. I also really liked our main characters; Uncle Nat, Hal and Lenny all felt like real people though I'd say the secondary ones felt more like card board cut outs.
Which takes me to the problems I had with the book. As much as I found the writing engaging and any time I set out to read one chapter I'd read six I felt like in the end the plotting let me down. This is a mystery plot that could have supported an adult novel (a thief is moving through the upper classes stealing jewelry and now they are on a train - jewelry starts going missing, is it the same thief? What are they on the train for?) I could definitely see the influence of Christie in the planning of the plot and setting here and kept having to remind myself this was a kids book and the plot couldn't get nearly as interesting as it would in her books.
But the problem is, that a child detective can't go into all the interesting layers an adult book would to try and obscure who did it. As a result this is a super striped down story that lacks any of the interesting twists and turns that make this "puzzle style" subgenre work. The kids never make any effort to get to know more about the individuals on the train - we get far more exposition about the train itself (which I loved) then we do about the other potential suspects - together the kids come up with one suspect, never look into anyone else, generally do almost no investigating, and when the red herring is proven innocent we're basically at the end. It honestly felt like the book was too long and an editor told Leonard they had to cute some of the mystery investigation or some of the train stuff and Leonard decided to keep the train stuff.
This leads me to my other problem, which is that Lenny is basically a pointless character. She jumps to the Red Herring conclusion and then the kids have no other conversation about it being someone else, never look into anyone else. And then, she's separated from Hal at the integral point - never gets curious about the obvious clue she has access to (that Hal eventually uses to solve the mystery), which btw is *very* out of character for her. I expected her to have gone through everyone's luggage. Hal doesn't talk to her about his suspicions, he never consults her etc... He just swoops in, says "I know who did it, you'll see..." and then goes on to solve the case.
I finished the book thinking, if I ever wondered if you could take a Watson/Hastings character and successfully blend them with a Mary Sue/Damsel in Distress trope now I know you can. Not that I'd ever wondered that. Or wanted that. I feel like the useless "partner" who just follows the detective around for narration purposes is a trope we've long outgrown and I'm confused why it was put into this book. What was Lenny's purpose? Train things? The story either needed Lenny to be a more involved detective (which would have matched her personality better) or leave her out.
Also, this is a modern book written to feel like it's written in that older style but we know it's modern times in the story. And we're going to have the police unreasonably jump to suspect the brown people on the train and we aren't going to at ALL mention that race is CLEARLY a factor in a police detective easily jumping to this unsupported, unsupportable, conclusion???? Because, I was SO waiting for Uncle Nat to comment on the obvious racism...
Honestly, in the end, while I enjoyed a lot about the book I felt like what I'd read was "Babies First Christie" novel where all the good parts about writing a mystery were stripped out and all that was kept was the basic structure of "locked room", one detective with a useless helper, and everyone gathers in one room to hear the solution. From an 11 year old. ok.
Btw. I knew what was up with the Red Herring from the outset, and with who. I knew who had done it from almost the beginning and I knew the diamond was being stolen as that scene was playing out. I get it's a kids book and I'm unnaturally canny with a mystery, but I read other kids mysteries that don't project the whole entire solution like this.
So. If you want a fun, hijinks style adventure with really enjoyable characters though a little lacking in development this is a good book. But if you wanted to read a MYSTERY I suggest you try Robin Stevens' Murder Most Unladylike series or Julie Berry's Scandalous Sisterhood of Prickwillow Place which keep the atmosphere and style this book was going for but actually do the mystery part well too.