Though Murder on the Orient Express is set on a train hurtling across Europe, America plays a central role in the novel. The Armstrong household, which existed in America, is temporarily recreated on the Orient Express. Additionally, we're constantly getting different views of the country from its cast of characters, including stereotypes galore. The Europeans have strong opinions about the Americans, and the Americans have strong opinions about the Europeans. At least, that's how they present their views to Poirot as he's working on the Ratchett murder case.
America, as portrayed in Murder on the Orient Express, is a land of opportunity and freedom.
Although the plot of this story has to do with America, it could not have been effective if it were set in America.