Penetration is a function of density. See http://edocs.nps.edu/npspubs%5Cscholarly%5Ctheses%5C2004%5CDec%5C04Dec_Kalinski.pdf for information on penetration of Whipple Armor (standard micro meteor protection).I would say that scattering a bag of plastic pearls in a counter-orbit should do the trick against any satelite/space station - that's probably something many states can achive if they want to.
I think (I have limited knowledge) that the penetration is a function of the impact energy released in the outer plate and the distance between plates. Below 3 kps the projectile is not vaporized and punches though as a solid. Above that the projectile is vaporized and the gas cloud spreads dispersing the energy across a larger area of the second plate so penetration is less. Above a certain velocity the gas no longer has enough distance to disperse adequately between the plates and protective effect levels off.
Denser projectile produce a smaller energy release in the outer plate due to the smaller crossection for a given mass and are therefore more effective, as shown in the study. They are also effective at higher velocities, which would support my rational.
Orbital velocity is ~8 km/sec. You only need to achieve escape velocity if you are leaving the earth-moon area, like on a voyage to Mars.Concerning the railgun; the escape velocity of earth is around 11Km/s, and such a projectile would be subjected to massive drag. Doubt that a rail gun can do the job.