Why don't the police use their own trains to patrol the railroads?

They did, for many years have special police cabooses added to high-value trains such as automobiles, Electronics and similar high value commodities when the train had a history of looting or need to wait in dangerous areas for legnthy periods of time. The Southern Pacific most notably had a series of cabooses specially decorated for "SP Police" that were put in the middle of high-value trains in the 1970's to prevent looting. Eventually, the added tonnage and maintenence of these cars made it inefficient. They turned to using HI-RAIL pickups (trucks with rail wheels attached to both bumpers so they could travel on the road and rail) to patrol some of the more troubled areas. As to their police force, they are fully certified and able to perform arrests, but they have no power outside of railroad property. Each rail company; Norfolk Southern, Union Pacific, BNSF,CSXT, etc maintains their own police department.