Dean, I'm not quite sure what you were describing, but it certainly wasn't a rail gun =). Its definately an intersting idea though, sort of like a multi-stage coil gun without coils being wrapped around a tube. Its worth looking into.
A rail gun doesn't even use a classical magnetic force. The force responsible for the motion of the slug is called the "Lorenz Force" (or something). What it all comes down to is that turns of an inductor want to push away from eachother. Now, a rail gun simply consists of two rails (solid bars or metal, preferrably silver for conductivity) and you just hook them up to some very big capacitors. Anything over 4kJ worth of capacitors will probably be pretty impressive. Next, you get a non-ferromagnetic slug (brass is good, for thermal and conductive properties) and inject it between the rails at some velocity greater than 0, otherwise it will spot-weld. The slug closes the circuit, and if the capacitors are even half decent, you'll get 100,000+A running through the whole setup for a few dozen microseconds. Thanks to our buddy Lorenz, the whole thing acts as a single-turn inductor, and the slug goes high into the sunset. The efficiency of a rail gun can be improved greatly by the presence of a vertical magnetic field parallel to the rails (above or under the firing track) but the field must be so strong that often the performance gains aren't worth the losses of powering hefty electromagnets above and below the rails (similar to the coil design you were speaking of).
Coil guns work simply by creating a magnetic field that draws the projectile toward the center of the coil. Optimally, the capacitors discharge before the projectile goes beyond the center, so it is not pulled in the oppisite direction. Coil guns generally achieve an efficiency of 2-4%, so you need big capacitors. Efficiency goes up tremendously in multi-stage setups, which is what Dean described, but using coils around the projectile instead of electromagnets below it. 20%-30% is achievable by a hobbiest with this setup.
For more info on railguns:
http://www.railgun.org/
http://www.powerlabs.org/
Coil Guns:
http://www.powerlabs.org/
**broken link removed**