Death on the Last Train by George Bellairs
First published in 1948, Death on the Last Train is a Chief Inspector Littlejohn mystery full of false leads, dead ends and old-fashioned charm. Detective Inspector Littlejohn of Scotland Yard is travelling to an assignment, exhausted after an arduous journey of delayed connections, when he catches the last train. A murder occurs in his carriage, putting on hold any other plans he may have had. The local police, out of their depth, commandeer the detective to help them solve the case. Delving into unrequited love, betrayal, and poison pen letters, Littlejohn must pick apart a tangle of grudges. Many men and women seem primed with motives, but which of them has it in them to kill?