You see this kind of stuff a lot on bridges and concrete walls. Graffiti has been with us for a long time and much of it is really trash... but some is obviously done by someone with some artistic talent and sense of design. But I never see a message or know what it means.
So I did some research and found out there is world-wide interest in the art form. There are many web pages devoted to this form of expression. That it is closely related to breaking (don't say breakdancing if you are cool!). And also sometimes associated with identifying gang turf.
This railroad overpass has several examples of the art form, and although many see the graffiti as vandalism or an eyesore, to me it adds a visual element to an otherwise drab structure.