Gaerwen to Amlwch many times!
Bangor to Pwllheli via Afon Wen (Yes, I do know that Bangor and Pwllheli stations are both still open!)
Colwyn Bay seafront miniature railway.
If Miniature lines are being counted I must include Tommy Mann's Express which operated in the grounds of the New Brighton Tower along the front at New Brighton - was a balloon loop line a mix between narrow gauge and minature. Closed when I was around 5 or 6.
Also Southport Pier Railway - though never travelled on the new and now closed tramway.
Also went on the Alton Towers line in the 1970s before it became a theme park that had a Baguely diesel.
I also have vague recollections of being taken for a ride on the Morecambe Pleasure Park railway as a youngster. I seem to recall the loco was quite a substantial diesel powered tank loco possibly 2-6-4? ran around and under the roller coaster if my mind isn't playing tricks.
Then of course there is the rather splendid set up at Dobwalls which aimed to replicate part of the Union Pacific and Rio Grande Railroads which I imagine many visited at least before it just became another theme park which stopped me going in the last 10 years or so of its existence.
Going back to standard gauge also rode the demonstration lines at Dinting and Carnforth Steam Town - though don't recall doing the Carnforth miniature railway.