I used illustrator to lay out my track design. Inkscape is a free version of Illustrator. I haven't used Inkscape, but the learning curve for illustrator is a bit steep and you are basically going to be drawing the track from scratch. You can change your stroke width to simulate lanes and get the appropriate distance between lanes.
The programs that I have seen that are dedicated to slot car tracks (Ultimate racer 3.0, and others) are focused on plastic track.
You could also just sketch the design out. In fact I would recommend doing that even if you use software as it gives you more freedom to come up with unique designs quickly.
If you do use Inkscape and need any help, I can try to help you out as best I can. I have many years experience in illustrator so I am sure I could help you figure something out in Inkscape as I have heard it is similar.
Best of luck,