Steam locomotive, usually called as in abbreviation; SL in Japan. Then, if you love SL or prefer nostalgic trains, Japan is also a place where you can enjoy watching or riding it. Anyway, Japan is not just a country of modern trains.
You can ride SL on four JR groups' companies, JR Hokkaido, JR East, JR West and JR Kyushu. Also four private railways also operate SL including Tobu, which is the second largest private railway company after Kintetsu.
However, not all companies operate SL as regular trains. Just Oigawa railway in Shizuoka prefecture operates daily basis. Other companies operate as extra trains on weekends or holiday seasons. Anyway, supplement is needed to take these SL trains.
If you just want to see SL in Japan, probably most easiest way is to visit Kyoto Railway Museum in Kyoto. However, single SL can be seen at local parks in Tokyo metropolitan area.