Subways are some of the best places to take pictures at. I like them because they have some of the most interesting variety of humans. You can find humans of all ages, colors, and styles. I also like subways because the lighting is always cool, subway lighting brings color to the images and makes photos more appealing. However, it is not easy taking pictures at subways, people are almost always moving which makes it difficult to have sharp pictures, but when they are sharp the outcome is awesome.
Subways are also representative of culture, people are coming or going places, most likely home to work or vise-versa and depending on your location the traffic and variety of people will change.