While these guns were theoretically capable of delivering ordinance to the areas around Calais their low rate of fire and the difficultly in accurate aiming using curved track made these weapons ineffective against moving targets like shiping in the English channel. Some bombardment of German emplacements was done, but the inability to spot the fall of shot at such long ranges made it impossible to determine if hits were scored.
Allegedly the only known hit scored by a British railway gun in WW2 was on Dover Castle when the brakes were not properly secured on the railway gun carriage, resulting in the gun setting off at some speed instead of the shell being propelled at the expected muzzle velocity.