Why are railway tracks always lined with Black pebbles?

Question

Why are railway tracks always lined with Black pebbles?

Answer 1

 crushed rocks were probably there to prevent plants and trees from growing between the tracks, and possibly impeding the path of the train. My dad felt that it had something to do with vibrations, and dampening them while the train moves. The idea is that the vibrations and force from the train on the track, spread across the hundreds of pebbles, leading to less strain on the tracks themselves and thus a smoother ride.