I love imagining how they pick their spot, choose their paint, carry materials to the destination. What time of day is it? How fast do these things go up? Are they working alone or with a group? How do they get up there? Over there? I love seeing a really detailed train car, and I love how they suddenly appear along the expressway randomly one morning and are just as quickly painted over with beige paint by some city worker. So quickly, usually, that I have no chance to snap a picture of them. Then it always makes me a little depressed that all their effort is just erased.
This one has been up (on the Ohio feeder ramp) for a couple of days now and the traffic was so terrible this morning I was actually able to grab it with my cell phone. This pic is crappy but click here to see more work by H2O/Reapa. It's so bright and detailed. I hate it that there's not more information out there about these people, but perhaps that's why it's so fascinating to me too.
This Gaper's Block article (from 2007) tends to come up when I do a search. If you find any of this interesting at all, something else you might like is this Chicago Street Art Flickr Group, or this blog.
If you want to take it a step further, and see a really crazy documentary on/by the UK's Banksy and this really random "film maker", rent Exit Through the Gift Shop.