something to do with tamping

I'm home today because I had a rehearsal this morning and a performance coming up in about four hours, so I figured rather than try to go to work for a few hours in between, I'd just take it easy for a day. I was just trying to take a bit of a nap when the windows started rattling. The floor kind of jiggles a bit sometimes when freight trains go by, but rattling windows is new, so I got up to check it out and saw this:

See right below the "CAT" where there's a pair of claw thingies going down into the gravel? The machine sticks those down, grabs a tie, lifts them back up, moves a couple of feet, grabs the next tie, etc. This description doesn't really help me understand what it does, but the drawing is kinda cute.

Ok, this article helps a bit...something to do with "tamping." If anyone does understand this, explain it in the comments. I know ballast is the gravel. I figured that out by some clever googling, after I saw a train made up entirely of cars sporting solar panels. (I couldn't think why a rail car would need electricity - something to do with opening the doors on the bottom so they can dump gravel on the tracks.)

UPDATE: Now there's another bulldozery-like machine out there pushing gravel around. My apartment is fun!