The Walker Evans title that you should probably seek out is "Many Are Called". Though Evans made his subway photos during a 3 or 4 year period starting in 1938, he was uncomfortable with publishing the photos until 1966 (lord only knows why). He used a Contax 35mm which he concealed in his coat, with a cable release that was threaded through the sleeve. He often rode with a decoy (Helen Levitt was one, there were others) to help conceal what he was doing. Though it's often stated that he "shot through a buttonhole", he actually shot through the opening between buttonholes (so his problem wasn't so much dim lighting as it was slow film).
I've made photographs on the San Diego Trolley (a couple are posted in the gallery) and found that using a TLR wasn't problematic in the least. I think that the move away from film and into digital happened so rapidly here that it's made it very easy to do this sort of thing in the open. Nobody really knew, or cared, about what it was that was perched on my knee. I think that to do this right and place a unique stamp on it, you have to use a different methodology. 35mm SLRs restrict your vantage point and because they don't look vastly different, they're still a recognizable camera, thus harder to conceal and operate.
I'm actually getting ready to publish my photo-book via Blurb. The version available now is really just a review copy. I've been going back and forth with one of my mentors regarding the current edit. You can take a look at it here;