There is archive film of these German guns firing. They definitely were practical, in that sense. Powerful hydraulics absorbed much of the energy of the recoil, although they did roll back along the track a few metres
As Paul says, Britain had a few too, although on not quite such a gradiose scale.
The Germans had a similar weapon in WW1 which bombarded Paris from somewhere beyond Verdun
Railway networks were the only way huge artillery like this could be mobile. Big static guns were used of course but to move them they had to be disassembled and transported (again by train) to be rebuilt at a new site - a case of weeks to shift position rather than days in the case of the rail guns.
Of course giant rail-mobile artillery requires that you have air superiority. These guns were either disabled by their crews and abandoned or withdrawn towards Germany once the Normandy invasion had taken place and Allied ground attack aircraft could fly anywhere over France with impunity.
