I had not realized that there was anywhere in North America that was "rat free". Would love to make Sunnyvale rat free but with all the old orchards and what not I doubt that is feasible.
The other reason that it is possible to maintain, of course, is the fact that the decision was made BEFORE rats invaded.
In truth, my house got a infestation of the smaller deer mouse [1], 2 years ago. But they are indigenous to Alberta and much smaller and easier to catch than the brown rat. It was quite easy to get rid of them, a cleaning in the garage and some traps did it.