Refers to a railway line or company that is privately owned, as opposed to public or JR lines. Common in everyday speech and news.
私鉄の駅まで歩いて十分かかります。
It takes ten minutes on foot to the private railway station.
この私鉄は通勤客でいつも混んでいる。
This private railway is always crowded with commuters.
I took a private railway in Tokyo out to the suburbs.
At this station, you can transfer between a private railway and JR.
New apartment buildings are increasing along private railway lines.