Using railgun as a mean of anti missile defense is utterly impossible unless major technological breakthrough occurs in guided munition. Remember, current unpowered guided artillery shells such as the M982 Excalibur has a CEP of 5 to 20 meters for close fire support use, and that is against a stationary, non maneuvering target. On top of that, sensitive electronic guidance components that can survived the massive acceleration force and the extreme electromagnetic environment simply does not exist at this time.