Unless there's a miraculous development in battery technology, I suspect that it will be several decades before we see a rail gun that small enough to fit in a single roadworthy vehicle, much less a backpack. Furthermore, as I've said in past rail gun threads, the current state of small high-powered battery technology would make such a unit the modern equivalent of the M2 flamethrower- almost as dangerous to wearer as it is to enemy troops.
OTOH I'm actually curious about the technology's potential in an anti-aircraft application. The ridiculously high muzzle velocity could solve the main problem with using traditional AAA against fast high-flying jets- limited range and the difficulty of compensating for wind drift. At ~8,000 fps, the projectile could conceivably hit most combat aircraft after only 5-12 seconds of flight time, even if the aircraft is flying over and past the battery as the gun is fired.