How do you get graffiti inside the subway?

I know this is really random, but I was noticing today on the subway that there is a lot of graffiti (how exactly do you spell that?) inside the walls of the subway. Not the main waiting areas, but actually inside the tunnels where the trains go. How does one get there to do that? Is it the workers, or do random people find their way into the tunnel to do graffiti?

*Disclaimer-I have no interest in actually doing this. Just harmless curiosity.*

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My grandmother, may she rest in peace always said she never like to ride the subway because of all the "giraffi" written on the walls.

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go to the subway at 3 AM