Although this thread is 2 years old, there is some correct information and some not so much.
The specs ARE called rail dust. It comes from 1 of 2 ways really. The first is during transit on trains. The iron/metal particles embed themselves into the paint. Another way is through braking. Iron particles are hot and do find themselves a home in your paint through that way as well. A lot of the time it is on the truck/rear of vehicles.
This is the hatch of our Magnum
The particles tend rust, which is what many people can see first. Removing them can be a bit tricky.
What I did was use detailing clay. After doing a spot about 6" by 6" on the hatch, my clay looked something like this.
Another way is to use a chemical cleaner with mild abrasives. I used Poli-seal, which worked very well on the particles that were not embedded very deep. The last resort for the sake of cost, or the first choice if you have it available is Iron Cut, or Iron X. A chemical that specifically targets Iron and will literally melt them off of the paint. Google it if you are interested.