16 2/3 Hz or 16.7 Hz as the newer standardization says is used by all countries that used AC early. 50 Hz wasn't feasible for railways before the 1960s or so.

16 2/3 Hz or 16.7 Hz as the newer standardization says is used by all countries that used AC early. 50 Hz wasn't feasible for railways before the 1960s or so. 16 2/3 Hz is at least in use in Germany, Switzerland, Austria, Sweden, and Norway. In Germany development started already in the 1920s and regular usage in the 1930s. France, Finland, and Denmark for example were later with AC and they chose 50 Hz.