Cruisers duel railway guns in Norway

How did Royal Navy cruisers deal with German railway guns that could hide in tunnels during the Norwegian campaign?

The engagements were awkward and uncertain; one British answer was to damage a tunnel so the railway gun could no longer use it freely as cover.

Cairo and Somali encountered railway guns around Namsos. A gun could fire and withdraw into a tunnel before a ship answered, turning terrain into a protective system. Somali appears to have addressed one case by damaging the tunnel, while the Cairo encounter ended less clearly and may have allowed the train to escape after taking damage. Naval gunfire against mobile shore targets was not a straightforward caliber contest.