I presume you're talking about that tubing? Jagwire has it in some of their cable kits. I ran across this with a recently acquired 1995 Trek 820 where the rear brake cable goes across the top tube, then around the seat tube through a metal tube. It originally had that plastic tubing all the way across and through the metal channel/tube, but I didn't want to use it. I used those rubber "donuts" on the cable along the top tube, but I could not get raw cable through that metal channel without fraying the ends, so used a piece of the plastic? Tubing to get the cable through there.