GarryGarret If you haven't looked at this yet, do that first.
The only other things I can think of (that aren't listed there) are A: Treat your rails like ions. (see guide) They will work the exact same. B: Get rid of some factories. You only need 1 ammo factory per railgun, and see here for the ratios you need when using cannons.
I will also make my own Re-fit version. Hope this helps!