The Orient Express was a long-distance passenger train originally operated by the Compagnie Internationale des Wagons-Lits on Earth. One Orient Express was built that travelled in space.
The two city names most intimately associated with the Orient Express were Paris and Istanbul, the original endpoints of the service. In 1887, Sherlock Holmes and Dr. Watson were riding on the Express through Austria when they were stopped by the train of Pope Leo XIII, who commissioned Holmes to investigate the disappearance of books from the Library of St John the Beheaded in London. (PROSE: All-Consuming Fire)
A space-faring version of the train was built that attempted to mimic the original in fine detail. (TV: Mummy on the Orient Express)
There was an Orient Express in India, and Clara Oswald wanted tickets for Christmas. (TV: Last Christmas)
The Eighth Doctor wasn't certain whether the cabins on board the R101 reminded him of the Orient Express or of the Hyperion, or "maybe" Storm Mine 4. (AUDIO: Storm Warning)
Donna Noble inadvertently gave Agatha Christie the idea of a murder taking place onboard the Orient Express (TV: The Unicorn and the Wasp)