Rat Bait?

Bait them with whatever it is they going after at the farms. Once you've located their runways, sprinkle whatever it is they're eating. Set up an over watch position and be ready. At the dairy I pest at I simply put down some of the feed pellets the rats are already going after. Not all rats come out at night, be prepared for some daylight action.
 
Creamy peanut butter spread over a white paper plate with some chocolate syrup in the middle.

The white paper plate gives great contrast to their dark bodies so you can shoot in low light conditions. I have a red LED light 4' over the above-described bait station. They get accustomed to the light, you have a controlled shoot, and they WILL come.