Subway Graffiti

My tiny green notebook is nearing capacity and I've begun to transcribe some of its tib-bits. I thought I'd share this one with You:

I wonder about the graffiti on the subway tracks, well not on the tracks, on the tunnels I guess, the metal posts that keep the streets above from falling on the streets below.Do the graffiti-ists do it just to know it's there? Or to some other end? Like harnessing the unconscious night-vision of straphangers reading off and therefore feeding their spray-painted sygil? Or is it just to have a piece of this city, to occupy it indefinitely publicly yet privately with a piece of their art, of their mind? Or is it simply to tell another person "I did this thing!" and to have that person think every time they pass on the subway line "The did this thing for me."?