here is another rebuttal
for an interesting node
at or close to light speed, any impact with atomic mass will cause problems. hell, gamma rays have problems
getting through a couple miles of nitrogen at less than STP (standard temperature and pressure). if you were going to build a railgun with the intent to use it in any
kind of orbit, you also need to have a plan for clearing out any atmosphere between you and your target. this would require a high output laser.
look at the whole earth/moon system as a subatomic particle would. you are flying REAL fast. REALLY REALLY fast. you want to hit the earth (the solid part), but you have this atmosphere thing between it and you. you try to go through. PINBALL! eventually you die because you lost all your energy hitting nitrogen molecules. the point: the faster you go, the harder it is to get there. this is the main idea behind einstien's view of relativity.
to kind of parallel that, think of humans and water. if you are in a 747 flying at 26,000 ft and head straight down into water, it feels like concrete. same thing with light speed and atmosphere (or anything else with mass). an aside to this is also what you see in frank herbert's dune with the shield suits the various atreides and harkonnen fighters wore. you could plung a knife slowly through the force field, but they were impervious to high speed projectiles like bullets.
now that i have that off my chest, i will get to the juicy bits. about the only feasible way to destroy a target from orbit is with a slow moving projectile of considerable mass. like an asteroid. or a house-sized cargo container like that used in The Moon is a Harsh Mistress (damn good book, that one...). without enough mass, a projectile will just go up in a puff of smoke; no matter how fast you shoot it.
UNLESS you had some super-dense material to throw through the atmosphere. like something out of a neutron star or a pulsar. then you could accelerate it to near light speed. only one other problem. trying to hurl that dense of material will destroy any kind of apparatus you use. square one.
another problem with the reverse-acceleration you have detailed above: what kind of a device will stand up to two opposing, simultaneous, high energy mass accelerations? not too many. maybe the moon. not something you don't build with a little bit of over-engineering though...