Stamboul Train by Graham Greene.
Aboard the Orient Express bound for Constantinople, Carleton Myatt and Coral Musker become entangled with fellow passengers in espionage, deception and murder. Graham Greene brings separate lives into dangerous proximity.
Read it for the temporary community a train creates. The novel asks how quickly private motives become other people's problems when everyone is traveling within the same enclosed world.
Published in the US as Orient Express. Penguin Classics paperback, ISBN 9780142437919, with an introduction by Christopher Hitchens. Cover image courtesy of Penguin Random House.