Private police forces are nothing new in the U.S. Railroads have had them for decades.
http://www.therailroadpolice.com/
Note: Completely useless information upcoming:
I've only seen RR cops twice in my life. The most recent they were sitting in their car between the tracks and a building. Don't know why.
The first time there were two of them looking intently at the device that lowers the arm to block the track, and then one of them turned and used his arm as a pointer as if something came from that-a-way. They both eyeballed me as I drove past, which was fair as I was doing the same to them.
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Racking my brain I cannot recall a single TV series based around RR cops. Not that Hollywood would do it right but they don't seem to have ever tried.