From 1965, this is a readable but unconvincing Solar Pons story, most memorable for its cameos by Hercule Poirot, the Saint and Bulldog Drummond. But their appearances are so brief and they do so little that their fans are not missing anything if they don't track down this story.
August Derleth did great service to pulp literature with his Arkham House publishing, keeping H P Lovecraft and others in print and in the public eye. He also wrote some historical novels which are respectable (although I personally haven't read them), but his adventure fiction is generally rather flat and lifeless. On several occassions, Derleth expanded a note or two from Lovecraft's papers into full scale books, but he missed the point of Lovecraft's existential, hopeless universe. Derleth reduced the Great Old Ones to mere alien monsters that humans could combat wih success.
With his long series of Solar Pons stories, he essentially was writing Sherlock Holmes-fan fiction, moving the stories up into the period following Holmes' retirement. The stories are usually versions of the famous 'untold tales' mentioned in Doyle's stories. Derleth was a lifetime son of Wisconsin, and his stories never convince the reader that they were written by an Englishmen. As a writer, he was competent but uninspired, without a real knack for surprises or momentum.
What is most interesting about the Solar Pons stories is the ways in which they differ from Doyle's Holmes stories. For one thing, Pons frequently met other writer's characters in pleasant little crossovers. Two or three times, he even found himself crossing swords with a tall, thin Chinese who had a brow like Shakespeare and a face like Satan...
"The Adventure of the Orient Express" takes place in 1938, when the coming war is hanging overhead like a thunderstorm ready to burst. For once without his detective partner, the narrator Dr Lyndon Parker is on the famous train leaving Prague, in the company of an abrasive and mysterious spy called Von Ruber. Also aboard are a bunch of Nazis, spies and adventurers looking for some hidden microfilm.
Most visible is 'a rotund little Frenchman* with waxed mustaches," who actually introduces himself as Hercule Poiret [sic] and manages to sneak in a reference to his little grey cells. Unfortunately, all he does is deliver a lukewarm warning to Von Ruber to watch his step. Even less notable is a "a chunky old man, whose eyes were masked by dark glasses, and whose mouth had a bulldog tenacity" (hmm....Hugh Drummond, I suppose?) And an elderly spy dies and is unceremoniously tossed out the train window. This is "Ashenten", a bit too old to be in active service but one who had done good work back in the Great War. He had been trained by 'old Colonel Somerset". This is a reference, of course, to one of the very first of the modern spy novels, ASHENDON, OR THE BRITISH AGENT (1928) by W Somerset Maugham, one of the more respectable mainstream authors and a great influence on later writers, especially Ian Fleming.
The best moment in the story involves an "almost flamboyant handsome young man" who passes Von Ruber and Parker a warning note signed with a little stick figure surmounted by a halo. A bit later, the young man gives Parker a friendly smile as he passes by, but he's now wearing a German uniform. It's a neat touch, the idea that Simon Templar is having his own Saintly adventure on the Orient Express that day.
If you really have a craving for a story with a Sherlock Holmes flavor, then the Solar Pons series are okay, but not much more. They're like episodes of a TV series, predictable and comfortable entertainment that don't ask much involvement.