On our local rail-trails, most people can hear a bike approaching from behind due to the crunching of the surface, and you will see most walkers or runners either move toward the right edge of the trail or else actually glance back at you. One situation that I then run into is when I get pretty close to someone and then get the feeling that they don't know I'm there. At that point, shouting out anything is going to startle them and I'm not sure what direction they might jump! So I have to decide if it's better to just go by them without any signal.
Note that I am also an avid runner who uses these same trails often and I usually run with an MP3 player going, but I have rarely been surprised by a cyclist coming up on me. In fact, it's faster runners who can sometimes startle me when going by!