These bells are replacing the mechanical bells because, they're much lighter, and that there are no moving parts.

A electronic chip embedded into the bell simulates a crossing bell, although not very well, from what I've heard. These types of bells can ring at 180 degrees, or 360. 180 degree bells are cheaper, but they aren't very loud and especially if you're on the opposite side of the bell. The 360s are louder, and can circle the whole signal.