A lot of great lines have been mentioned. Here are a few more:
In Wales: The Vale of Rheidol, the Brecon Mountain, the Snowdon Rack Railway
In England: The Lynton & Barnstaple
in France: the Vivarais, the Friossey (probably atrociously spelled)
In Argentina: the RFIRT, and another "shout out" for La Trochita (the whole line)
In Brazil: The former EFOM line out of Sao Joao del Rei, the EFPP, the Dona Teresa Christina
In Columbia: The narrow gauge train that runs out of Bogota
In Equador: The Guayaquil and Quito
In Cuba: whatever is left running - Raphael Freyre?, Mal Tiempo? Espartaco?, Simon Bolivar?, Obdulio Morales?
In India: the Ooty line, the Matheran line, the Gwalior line
In Burma: the Burma Mines Railway
In New Zealand: the Rimutaka line (if any part of that has commenced operation)
In Australia: the Bennet Brook Railway and the Walhalla Goldfields Rwy
In Germany, Austria, Switzerland, Poland, Romania, and Eastern Europe as a whole: there are lots of 600mm, 750 mm, and 760mm lines,
my favorites are probably the Cranzhal to Oberweisenthal line (the Fichtelbergbahn), the Murtalbahn, the Dollnitztalbahn, the Zillertalbahn, the Achenseebahn, and the line on the Isle of Rugen
Finally, there is a recently-restored in line Eritria that uses 0-4-4-0 tank Mallets, but I am currently not remembering the name
Glenn