I liked the fact that so much had been done with some coloured boxes and a few props. And crateman's creators had obviously taken some risks to install him. Here was street art with a sense of humour and a dash of courage; a whimsical, Lego-like presence enlivening my train ride. Last week, feeling faintly ridiculous even by the standards of cyberspace, I Googled the question: "Who is crateman?" I didn't exactly find an answer (and perhaps there is none) but I discovered he had quite a profile. He has been turning up around Melbourne for several years now. There's a crateman website, made by a fan, and a YouTube slideshow.

Most of his earlier cameos are long gone, but they live on in photos. I saw crateman perched on the roof of a Richmond building and hanging, as if crucified, from the side of the Newport Mill. I saw a weightlifter (cratelifter?) hoisting a plastic barbell and two crate men ascending a city skyscraper, one leaning down to offer the other a helping hand. I read of forays into Bendigo, where in August 2007, ABC radio reported the arrival of a huge crate person in the middle of the town's busiest roundabout. By lunchtime, he had disappeared. A year later, a giant made of more than 40 crates had turned up under a Bendigo gum tree. A sign announced: "It's been whey too long! It's crate to be back. Crateman."

That sign was a mistake, I reckon. Part of crateman's appeal is his enigmatic silence, the way he keeps us guessing. He can, at times, resort to painfully undergraduate obviousness (such as the four-crate erection on one prone figure) but at his best, his character's body language is surprisingly sophisticated, conveying mood and meaning.

The best of crateman may be a four-person installation that appeared on a sound barrier at Victoria Park in 2007. Like frames in a cartoon animation, the four figures are all at different stages of climbing this wall: from just hanging on, to fully seated on top. It's a complicated piece, using more than 150 crates, which must have been hard to pull off. So who is crateman? The only clue I found was on the website www.streeteditors.com, which reported that these large-scale artworks "subverting built forms" were created by "the mysterious Sam, Jerome, Ed and Gab". Actually, I'm glad I learned nothing more about crateman's makers because this way I can fully indulge my own readings of the work.

Crateman is part of a thriving international street art scene. In New York, the artists known as Skewville have dressed up as construction workers and built a false fence outside a public park. In Sydney, a group of "guerrilla knitters" apparently drape strange woollen forms on street trees and fire hydrants. In Washington, artist Mark Jenkins has used Scotch tape to turn parking meters into "lollypop" sculptures and created life-size "tape men". Given his city's tough anti-graffiti stance, this medium, he says, is less risky than spray paint. "There aren't laws specifically designed against what I do".

Crateman might partly be a response to Victoria's hardline graffiti laws, which virtually demonise spray paint. His impromptu appearances also prompt questions about the use of public space and the kinds of officially sanctioned works that get commissioned as public art. Maybe I'm reading too much into some plastic stacks. But I hope he sticks around. www.cratemen.com