Let's pause the theorycrafting for a second and look at where we are coming from for some context.
Spinal mounted weapons, at their essence provide
* Increased range and damage
At the cost of
*All the normal penalties inherent in increasing range/damage
*Only being able to fit 1 on your ship.
What is important to note is that spinal mount weapons do not alter the rules. All their calculations are done in exactly the same manner as a normal hull mounted weapon. Rate of fire, damage, range, everything is calculated exactly as if they are a weapon of exactly their size, the only change is that the size limitation is increased by 25%/50%.
With lasers, increasing the focal lense size by 25% increases the base damage, which in turn increases both the range and the energy consumption (which could result in reduced rate of fire if capacitor tech does not keep up).
If we apply the same approach to railguns, the solution seems simply to increase the calibre by 25% in a similar vein. Base damage increases which again causes an increase in range and energy consumption, all following the same rules as hull mounts.
Is there any reason railguns were not originally allowed to be set as spinal mounts under these rules?