As others noted, this is an artifact of the time in which railroads were created. Railroads are unusual from a legal perspective because they stretch across many jurisdictions, and it is not always immediately obvious which one should apply (e.g., a train in motion across multiple city/county/state borders - by the time a local force can respond, it's in another jurisdiction). Giving them their own force, with arrest powers limited to railroad property, allows them to keep order on their own territory and figure out afterward what jurisdiction should take the offenders.