This is the most basic rule of subway/sidewalk/biking/driving etiquette, and seeing people break it makes me want to rip the bedazzled cellphone out of the offender's hand and hurl it on the third rail in the hopes that they will dive for it headfirst. It breaks down like this:
Just as if we were driving on the highway, slow traffic stays to the right, and people pass on the left. The people on the right are staying over as far as possible, close to the shoulder, and are therefore doing what I like to call "Hugging the Rail".
(I think it comes from when I used to go to skating parties in middle school, but, being someone who did not skate that well, I usually had to spend the first 90 minutes or so scooting myself along, hand-over-hand, pulling myself around the rink using that little waist-high wall that separates the smooth wood floor of the rink from the cheap, thin carpeting near the arcade games, skate rental area, and nacho counter. You know who was a good skater? Josh McGraw. I remember him getting all the girls, too, as he was athletically gifted in every way. I realize NOW, of course, looking back, that he had red hair and freckles, which means he had no soul. I bet this is why he was such a good rollerskater.)
This is the natural order of things, and is even mimicked on escalators for the subway systems of both Washington, DC and New York (is this true on any others, besides Chicago as well?); you stand on the right, and walk on the left. Slow people keep right. Hug the rail.
This should also be true on sidewalks, subway stairs, and for bicyclists. All too often, however, that is sadly not the case. Instead, I can't tell you the number of times I get off the train and head down the stairs to go home, and I have to kick the person in front of me in the small of their back because they're busy sauntering along, deliberately slowly so as to not lose their balance while they're TEXTING and walking down the center of the stairs at the same time.
Another example would be a woman with a MASSIVE PURSE sticking out behind her, situated on the subway platform in such a way that she is equidistant from both the front edge of the bench behind her, and the tracks in front. The problem being, of course, no one can get by her. A simple, "Excuse me" will usually suffice - until you get close enough to hear that she has her ipod on full volume. And have you noticed it's almost NEVER any good music? It's always either that horribly generic Latin beat that blasts out of taxicab windows passing my apartment in the middle of the night, or some shitty Euro-techno that is probably only appropriate for someone still rolling on Ecstasy from the night before. In that case, the "Excuse me" is no longer an option. Your only recourse is to avoid bumping into her MASSIVE PURSE as you attempt to sidestep behind her, likely whacking your shin or calf on the bench, thereby creating a resentment for the rest of the day against the MTA, the purse designer and Latin musicians (to be honest - I generally have a resentment against that latter group, anyway).
The last fine example, and there are many, is the person who stands with their back to the open subway door, but has no intention of leaving at the next stop. The way I see it, there are three types of ways the human body can end up, when fully extended: 1. Horizontally, like a carpet or a pizza; 2. Upright, but profiled, like a book on a shelf; and 3. Vertically, but head on, like a picture on a wall or, appropriately, a wall. Too often, people are standing in subway doors like #3, while they SHOULD be standing in the doors like #2: sideways, with plenty of room to get by (hugging the rail). Instead, I have to push up against them and try gain a valuable few inches of floorspace, while THEY'RE probably thinking I would like to take them out to dinner and a movie so we can get nice and cozy at my place after, but minus the dinner, movie and my place bits.
All of these instances, and the myriad of other examples, would easily be avoided if people would just HUG THE RAIL. Move to the side. Go at your own pace, but please leave room for people to get by. People who, while not necessarily LATE, are people who have decided that, if walking counts as exercise, then dammit I'm going to be the best, fastest walker ever, especially because, in NYC, walking is sometimes the only way to get from Point A to Point B, and often there are hot dogs at point A and ice cream at Point B, so step aside, asshole. Hug the Goddamn rail.