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.

It's from a time when the country was more sparse. A normal citizen can make arrests only with proof of a crime, in some very important situations like theft or trespass you'd like to be able to arrest on suspicion.

If anything these hyper-specific forces tend to be more professional that local police forces.

I don't think it's that weird. Our national railway in The Netherlands also has a security force. I haven't looked at the differences in authority between them and the UP police force, though.

The railroads in the US are pretty heavily regulated by various US government departments. It makes sense to have specialized police for the needs of the railroad.