And Then There Were 59: The Mystery of the Blue Train by Agatha Christie

Tell people you're reading a Christie novel set on a train--a
Poirot, no less--and they'll almost definitely think you're reading
Murder on The Orient Express. And you might be! But you might also
be reading The Mystery of the Blue Train, which predates its much
more famous sibling by a few years.... Climb aboard with us and
traverse not just the French countryside but a hybrid
whodunit/thriller centered on murder, adultery, priceless jewels,
and a 33-year-old woman who is apparently in the "autumn" of her
life (!!). This one definitely was not Christie's favorite, but we
found a lot to enjoy here.