A long time ago, JR was government run. But now it has private management.
「JRは昔国鉄でした。今は民営です。」は英語で「Japan Railways (JR) used to be the Japanese National Railways (run by the government). But now it's under private management」という表現が良いと思います。
「A long time ago, JR was government run. But now it has private management.」という表現も言えます。
JR used to be a national railway, but it has been privatized.
この場合、次のような言い方ができますよ。
ーJR used to be a national railway, but it has been privatized.
「JRは以前国鉄だったが、民営化された」
a national railway で「国鉄」
to be privatized「民営化される」
ご参考まで！
アメリカ合衆国 
カナダ 
