“4.50 From Paddington” by Agatha Christie

“I,” Sam said, “am married to an Agatha Christie addict.”

Why, yes, Sam, I believe you are. 

Unfortunately as I write this it is two weeks after I finished reading, and I'm really struggling to remember this book. It's the one that was also published under the title What Mrs McGillicuddy Saw, and it starts with a friend of Miss Marple's boarding a train. On that train ride, Elspeth's train briefly runs parallel to another, and during those moments, she sees into a compartment on the other train--where she witnesses the strangulation of a stranger. Mrs McGillicuddy is both horrified and perplexed--what can she possibly do?

Well, she does what any friend of Miss Marple's would do: tells Jane all about it and asks for her help. Miss Marple is able to make a few deductions on her own, but she reaches a point where in-person investigation is required, and her rheumatism won't allow her to undertake this personally. She enlists the help of the most efficient Lucy Eyelesbarrow. 

Beyond this I have very little in my notes (all that remains is "Misdirection" and "Poor Emma") and what little else I can remember would be a spoiler to relate. I'm sure I did not guess the solution or I would have written that down. I'm also sure I enjoyed reading just as much as I expected to (I only would have made a note of it if I hadn't enjoyed it).