What are the rocks and pebbles underneath train tracks for?

Besides the usual media sources, questions and thoughts from my friends occasionally serve as inspiration for my blog posts. Recently my father brought up a question which has remained on the table for a quite a while now, while I'll now address.

What is the purpose of all those tiny rocks and pebbles that one sees laid underneath railway tracks?

He brought up this question, while my family and I were spending the day in New Hope, PA and happened to come across the historic New Hope train station, shown in the picture below, as we were ambling about. So what are all those rocks for?
In that conversation, I surmised that those 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.

Turns out we're both correct (though his explanation is obviously a lot more clever). A quick glance through the Internet reveals that those pebbles, rocks, etc are called track ballast and according to wikipedia, they are...

...used to facilitate drainage of water, to distribute the load from the railroad ties, and also to keep down vegetation that might interfere with the track structure. This also serves to hold the track in place as the trains roll by.

Some other interested soul on Yahoo Answer's gives a couple of additional advantages to using ballast...

1. It prevents the rails from moving sideways which would be the natural tendency around most curves - the train would normally push the lines outward and the ballast stops this happening for the most part.
2. It is an easy way to make a level running surface for trains - special track tamping machines are used to re-pack these ballast rocks around and underneath rails where they have been pushed out by the constant passing and vibration of trains. Much easier than trying to make a completely level track bed on the earth, and cheaper than using concrete beds all the way too.

Right. So now we know...