I went through and took a look at the ones you have posted on Flickr. I know exactly what you are going through (I think) as I've struggled with photographing graffiti art as well. First off, a few of them look like the subject is the box car or the train which just happens to have graffiti on it rather than the graffiti being the subject. The other challenge, which is also one I've struggled with is how to make your photograph the artwork and not simply a copy of the graffiti. Sorry, I'm probably asking more questions than I am answering but it's the way my brain is working today.

Hope this helps a little bit.