I ride with a guy who fell on one of trails. He ended up braking his leg. He recommends taking a whistle with you 'cause you can make noise for a longer time period and at a louder level when someone is looking for you.

I ride alone a lot. But the trails near me are relatively well used. The exception being in the dark and winter. I am a patroller so I normally take a bunch of stuff with me. Even when I don't take a bunch of stuff, at least a tube and cell phone.