We had rats something terrible in the chicken coop this summer. When I went in there after it got dark, about a dozen or so would jump out of the feeder and scurry away. They were eating more chicken feed than the chickens!! Disgusting creatures. I shot one or two, but that was it ... they are fast ... and they multiply like rabbits. It felt like I was attempting to empty a swimming pool with a tablespoon during the rainy season.
I finally picked up a plastic bait station from the feed store and about a dozen pouches of the pelleted bait. The bait station is made to be secured to a wall, plank, board, etc. It has small openings and baffles inside so that just rats and mice can get in and the animals you don't want to kill (in this case our chickens and cats) don't upset the station.
I set up the bait station, secured it to the wall, filled it with the bait and eagerly awaited getting up the next morning to check it. In the morning, not a single bite of the poison was touched. Next morning, same thing. It took over a week for the rats to finally start taking the bait. Once they did, I had to put two pouches in every morning for a little over a week before I got them all.
Rats, unlike mice, are extremely intelligent. After seeing first hand how smart they are, it's no wonder why they are the most populated mammal on the the earth. They are very wary of anything new placed in their surroundings and have a very keen sense of danger.
If you choose to try rat traps, you must first place them out unbaited for three days. Then bait them, but do not set them for three days. Check the traps every day to see if the bait has been taken and replace it with fresh. Finally, on the sixth day, set the traps. That is the only way to catch them with a snap trap.
I wish you the best getting rid of them, they are not fun to deal with!