While not narrow gauge, a journey on the Chihuahua Pacific Railroad should be a "must ride" for any railfan. I can't think of anything in my lifelong railfanning experiences that has come close to matching the excitement of my cab ride thru the Urique Canyon and Tomoris areas.
Pardon the poor quality of the attached image, but the Tomoris trackage includes three elevations of rail line plus a 180 degree spiral tunnel.
It just doesn't get any better than that!