There are criminals on the metro. People who drink from cups without a lid. People who eat and play loud music. They disregard the signs. People who don't get up for people in wheelchairs or old people. And then there are the worst: the people who think that they can just ride the metro for free, but they can't. They get on without paying for the ride. The security guards have a trick to catching those people. They get on just when the doors are about to close so you can't escape and then they try to check everybody before the next stop. It's a hit or miss. You might be lucky and get a free ride or you might get trapped by a security guard. They have whistles and badges and everything, so beware. And play fair, pay your fare! :)
Here is some metro fashion. This guy got on and the first thing that came to my mind was "Hey, more power to you!"