Do you mean closing the switches? Or opening them as the missile passes given points in the fields?
I designed, but never built, a tiny one for throwing a 3/8" section of 1/4 inch aluminum dowel, using braided "solder wick" for contacts instead of rails. But my son borrowed my big caps and I haven't seen them since. I also intended to use the big neo magnets from a voice-coil driven hard drive for the mag field.
If I had got the thing to even eject the aluminum rod, I would have considered it a great triumph.
I was pretty sure I got the right-hand-left-hand motor and generator rules mixed up, as well as "north-seeking" versus "true north" poles of the magnet mixed up, but I figured there were only four combinations to try in order to get it right.
Might have shot backwards on one combination, though. 
Still, a micro-triumph. Yay, Terry!
Terry, 230RN
I mean opening. It's not a capacitor, it's an inductor, so to transfer energy to the load you need to "open" the inductor, while the projectile closing between the rails closes that part. Think of it as a power supply, an inductor, and a load (the two rails with the projectile between them), all wired in parallel. What you do is open a switch on the PS, while closing the load with the projectile.
You don't need any magnets or external magnetic field to make a railgun--the current loop (down one rail, through the projectile, and back on the other rail) creates a field inside that loop--the interaction of the current through the armature and that field is what propels the armature down the rails (it also puts extreme forces on the rails trying to push them apart--like several million pounds).
