If you ask a local what the number one unwritten rule about metro is they'll say "stand on the right side of the escalator!" It's true, most regulars will stand on the right and walk on the left. If you're standing on the left, especially during rush hour, you will be run over or promptly asked to move. Metro at least recognizes how this irks regular commuters and has made some feeble announcements telling newbies that "most people stand on the right". Unfortunately these announcements, if even audible, come once you have already taken an escalator into the station.
Let's take this one step further and follow the "stand right, walk left" concept your entire metro experience. This means if you need to stop, or you're moving slowly digging through your bag for the farecard you thought you put in a convenient place but now can't find, kindly move off to the side and let those who just want to get their trip over with move smoothly through the station. Karma will thank you later.