Most of Agatha Christie's titles are descriptive advertisements of the plot: Murder on the Links, for example, is about murder on a golf course. Seems pretty straightforward, right? That's because these novels are tied to a distinct genre (mystery) with a set of conventions (plot = murder). The publisher and author want these books to appeal to a mass commercial audience, and part of doing that is making sure that the buyer knows up front what he or she is getting.
Interesting note: Christie's novel was originally titled Murder in the Calais Coach when released in America, so as not to be confused with a Graham Greene novel with a similar title (and plot). We guess stories about murders on the Orient Express were pretty big during this time period.