I agree that both the Welsh Highland and its sister line, the Ffestiniog Railway are "musts" for the narrow gauge enthusiast. Book the Pullman observation car on the WHR. Pricey, but worth it. While you are in tNorth Wales, fly over to the nearby Isle of Man where one can ride an interconnected network of narrow gauge steam and electric lines - including the 2-foot Groudle Glen Ry and the mountain climbing Snaefell Mountain RR with its Fell center rail.
However, for the most spectacular narrow gauge rail experience in Europe, nothing beats the meter gauge Dampfbahn Furka Bergstrecke (Furka Steam Cog Railway) in Switzerland. The DFB is an 11-mile rack line with 12 percent grades and a mile-long summit tunnel.. It crosses the Alps at an elevation of 7,100 feet, making it the second highest through railway route in Switzerland. DFB connects with the meter gauge Furka Oberalp (FO) at both termini - Oberwald and Realp The DFB is actually a former FO route that became redundant when a lengthy tunnel bypassing Furka Pass opened a while back Before the tunnel opened, service over the pass had to be suspended each fall. the electric overhead taken down and one of the bridges disassembled and stowed away due to avalanche danger..
The DFB is operated by enthusiasts and has a steam rotary and several 2-6-0T rack engines used by the FO prior to electrification.