What kind of people take greyhound buses?
The kind of people that ride buses. You know men and women, sometimes kids.
Actually, a lot of people take trains between those cities, because they're much faster than the bus and aren't really that expensive.
People who don't like trains and planes and some because they are short on cash.
I take the Greyhound when I don't have enough money for car rental, Amtrak or plane.
People living in or visiting the USA.
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I can only speak for Boston <> NYC. You see all classes of people on the bus between Boston and NYC. The bus cost $40. The train costs over $100, with some exceptions. Both the train and the bus take 4 hours, there is no advantage time wise to take the train unless you take the Acela which is guaranteed to cost over $100.
The east coast has a pretty sophisticated rail system. So the dregs you see on Greyhound are usually those traveling west or traveling through the west.