The Federal government generally stays away from general police duties, because most of the laws typically enforced by the police are state laws. When the Feds operate police forces, it is scoped to a specific institution or place. The VA, GSA, US Mint, etc operation police forces oriented towards federal property.

I think it makes sense for railroads to have police, as the crimes that affect them are multi-jurisdictional by nature and specific to the railroad. Colleges are similar in many ways... US Marshalls + pinkertons provided security to railroads in the 19th century. Professional police alleviate many of the abuses that inevitably happen with security people.

I get a little uncomfortable with private police not tied to an institution. When I was in vacation in South Carolina, the resort rent-a-cops were sworn officers... that sounds like a way to cover up crime to me.