Interpretation of subways is based on your experience with this mode of transportation.
If you regularly use the subway, you may need to look at the other parts of the dream more carefully.
If the subway is not a regular part of your daily routine but is the primary dream image then you can consider it to be symbolic of the way that you navigate through your own emotions and hidden parts of self.
The subway is under the ground and represents the unconscious parts of self.
All of us interact on conscious and unconscious levels.
We have intuitive feelings and unspoken understanding that direct our lives as much as the obvious and fully conscious things.
Consider the subway ride in your dream and try to understand what it is conveying to you about your more subtle navigation through ongoing life experiences.
See also: Train
Dreams about traveling on the subway are highly symbolic.