A rail gun basically means the slug is accelerated by high powered magnets instead of explosive charges. These magnets used to turn on and off as the slug passes them along the barrel. Later in development they changed it to one magnet that moves with the slug constantly accelerating it (see wikipedia for a better explanation).
No explosive charge making the gun safer but very costly (energy wise) to fire. Also the slug itself usually has no explosive in, relying on its sheer speed to destroy anything it hits.
Search youtube for railgun for some examples.
They could have done some minor changes to the way the gun works and the type of magnet but Its much more probably that they have outfitted the ship with high charge capacitors (batteries).
1 capacitor per shot, charged over time from the ships engines they are probably designed as 1 shot ship gutters. Sort of guerilla warfare style, or backed up with the rest of a fleet while the capacitors recharge.
Either way, I doubt there's any big differences to the gun just the ship will be better designed to compensate for the 1 big gun.
Oh and they are willing to broadcast it because even if they are expensive, who cares, they have 3!!
In simulations the only ships capable of beating a railgun armed destroyer head to head are an aircraft carrier and submarines. Again, like I say put it with a fleet, a few cruisers and a carrier and if you can get it into range then 1 shot will sink aforementioned carrier worth £X million.