Constantinople was a very cosmopolitan city in January of 1923, the month that is the intended setting for Horror on the Orient Express. However there was considerable anti-British feeling, founded in Britain’s role in the Western powers’ military occupation of Constantinople (which lasted until the first Turkish troops entered the city in Constantinople in the October of that year) and the perception that Britain took the Greek side in many of the so-called “Eastern questions” of diplomacy.
This is a tumultuous time for the imperial city. The Sultanate had been abolished in November 1922. The Treaty of Lausanne, which would settle the question of sovereignty, was not to be signed until July 1923. Meanwhile, Mustafa Kemal, Ataturk, played brinkmanship with the European powers.
The resultant potent mix of nationalism with political and military expediency sometimes manifested itself in some bizarre confrontations.

Kemalist Activities in Constantinople, the ‘Singleton Argus’, 20 January 1923 [Source: the Australian National Archives]
This article was once again unearthed by Darren, our Stalwart Historian.