I had to replace the cable on my Rigi tram and found that black nylon string about 1.2 mm works for me. You can see it in action starting 13 minutes 45 seconds into a video of my railroad on youtube:.The video was made by Bud Harrington, a fellow member of the Puget Sound Garden Railroad club in Washington state. the video is at:
After posting I see that youtube link is blocked. So you can go to Youtube and search for Sunny Bay Railroad.