My only suggestion is to wrap the line with foil. I have heard they don't like the texture of foil and are reluctant to chew it.

In the meantime, get an empty 5 gallon bucket and loosely drape some window screen or nylon net material over the top. Place the bucket as close to the rat racks as you can so he can reach it easily. Very carefully place a shallow container of water along the rim of the bucket farthest from the rack, so he must cross the top of the bucket to reach it. The idea being, when he goes for the water he can see and smell, his weight will collapse the screen and fall into the bucket. I cannot guarantee he'll stay in there long, but I've used this with success before.

My only other suggestion is one a friend has used, try setting out a shallow dish of beer. According to my friend, the rat will consume enough of it to slow it down and allow you time to grab the rat by the tail.

Gale