Considering that the title is half as long as the entire poem, we might as well put it under the microscope, as if it were part of the poem. The "metro" is short for the French word for "metropolitan" and refers to the underground subway system in Paris. At the time that Pound wrote this poem, Paris was considered the most modern and sophisticated city in the world. To have a subway was a big, big deal. It meant that the city was "advanced" and "civilized" compared to all those unsophisticated cities without subways (scoff).
But subways were also scary. Before the age of mass transit, people really didn’t come into frequent contact with strangers. The subway, though, brings an individual into contact with many, many strangers. You sit across from someone for a few minutes, then you or she gets off the subway and most likely you will never see each other again. Many people considered, and still consider, this phenomenon quite bizarre.
The title begins with a preposition: "In." This is the first sign that the poet means to put us, literally, "in" his shoes. We’re going to be down there, in the metro, seeing what he sees.
In a poem this short, every word is important, even articles like "a" and "the." The poem takes place in "a" station, but the poet isn’t going to tell us which one. (In fact, it was the station called "La Concorde," but Pound obviously didn’t feel that giving the name would add anything to the atmosphere of the poem.) It’s meant to be vague; the situation is so common that it could be any station. But it can’t be any metro. There’s only one metro, "the" metro. Pound wants us to think like a Parisian, so we can’t be confusing this metro with, say, the New York Subway or the London Underground.
Pound could have just called the poem, "In a metro station," but he chose to make the title longer. There are lots of reasons he could have done this – it sounds really cool, for example – but another reason might be to highlight the word "station." A "station" is a stopping-place, a place where things stand still. The poem itself is a kind of station, because it freezes all the bustle of the metro and the crowd into one lasting image. So the title isn’t just referring to a place where subway trains stop; it’s also talking about how the poem works.
Finally, there are the "Stations of the Cross" from Christianity, which are the prayers said to remember the stages of Christ’s journey carrying the cross on the way to his crucifixion. Not to say that the metro is a place where people are crucified (we should hope not), but only that the experience described in the poem might have religious overtones.