It doesnt have ammo. The railgun needs cannon ammo to fire.
Also, the reload time will be very long, because you have them running from the bottom to the top with power to refresh it - and the shields are sub-optimal with their distance from the reactor. Both these problems can be solved with power storage, therefore not exposing the ship to extra explosions.
Also also, try and have more than one railgun if its your only weapons, especially when they are only that long.