With all the grime, mud, salt, and general grossness visible on the floor of the metro, you'd think we wouldn't have to breach this subject. Unfortunately, a recent ride on the train actually prompted this post.
flickr) beautifully illustrates the area of the train where this happens most often. On the right and left of the image, see those radiator-type boxes on the floor? They distribute air in the train, but for some people they're an invite to 3" of butt space. Then when that gets uncomfortable, they just scoot down to the floor.
First off, I should not have to say this about the floor, but it's disgusting. I wouldn't want any part of my body besides the soles of my shoes touching it. Second, if you're sitting on the little air distribution boxes, it's going to get uncomfortable, especially since you have to sit slouched over to fit under the grab bar, which people would love to use if you weren't blocking it. Third, I can't guarantee that the ledge is not as disgusting as the floor. Why would you want to park yourself on it?
But the real problem comes down, once again, to space. If you're sitting down on the floor of the train, that probably means all the seats are taken. Which also means the train probably has people standing. And no matter what, there are people entering and exiting the train. If you're sitting down on that ledge, you're taking up at least two people's worth of floor space. If you spread out your bags around you, that's even less room for others to share the joy of a metro ride. And finally, when people try to get off, at best you're blocking a possible route to the door and at worst they'll need to step on your legs, feet, hands, bags, etc to get to the coveted open door.
On a regular, partially empty (and by that I mean only 2-3 people standing) train we'll tolerate your floor-sitting. But when things start to fill up, or when a lot of people are entering and exiting, please share your floor space with others. We'll thank you by not stepping on you or your things.