I was doing the train photo thing just this evening with a, Shock - Horror! Digital SLR. Nikon D100 with an old manual 50mm lens (that disables all auto anything functions including the light meter). So with the crop factor it is an 80mm or so lens(you know what I mean). Tricky for off the hip shots, I have a little series of a girl an her boyfriend from the neck down. Not what I was aiming for. Anyway the Nikon is pretty quiet for an SLR but still I would suggest using the technique described earlier; waiting for a lot of noise.
I didn't and was doing another sneaky shot with the camera on my lap (me sitting) and as a woman walked past (where I thought I had aimed) I pressed the shutter. Slap bang! Too loud!
She, of course, heard it and gave me a bit of an earful. I quietly told her that I was doing nothing illegal. She said that she did not want me to take her photo. I said OK I will delete it. She left. I looked at the screen and I had taken a magnificent shot of the ceiling. No woman at all. She's probaby still fuming at the nerve of 'that strange man'. Pity I didnt have the chance to show her the shot, she may have found it funny. I did.
Later I was taking a photo of the stairs in the station and took a shot with the feet of a couple of people walking down them. They too confronted me, wthout hesitation (that's a little surprising for me, in itself, as I have been recently described as a little bit scary looking).
This time I was just standing there obviously taking a photo (with a stupid big camera admitedly - why are they so BIG?). Their confrontation with me was much more pleasant as I had made all effort to be obvious. I also offered to show them the photo which was when they realised they didn't really care if I took their photo or not.
If I was using a wide I believe it would all have been easier as you can look like you're not taking someone's photo when they are nicely placed in the side of the frame.
I'm 180cm (6 foot) tall and it is not possible for me to blend in. A lesson learned in one evening.