No longer just the province of college students, you'll find backpacks on schoolkids, working adults and sometimes even elderly people. Problem is, people often forget they have them on. Or forget how much space they take up behind them.
If you're hopping on a crowded train with a backpack, make sure you don't whack somebody with it when you turn around or back up. Scored a seat? Place the pack on the floor, between your legs, and as close to your body as possible. If you leave it protruding into the aisle, someone's bound to stumble over it. And for heaven's sake, don't set it down in the center of the aisle, as convenient as this may be. Neither put it on the seat next to you, unless you paid for two seats or the train is empty. These rules, incidentally, also apply to other bulky items, such as strollers, suitcases, packages and large purses [source: Seid].