When working on my model railroad layout, I usually mark where the roads will be using a sharpie. I've always used the old trick where you use a piece of compressed foam and poke two holes with the desired width in between them to mark two parallel lines to make your road.

I wanted something a bit more precise and adjustable so I designed this simple tool in Tinkercad. You just need a screw, a washer and a nut to tighten the two parts together. Slip in two sharpies, adjust it to your desired width and make parallel lines!

I have added two holes in case one wants to tighten the sharpies in their holder but after using it for 6 months I found out I don't need to tighten them anymore than they already are just by inserting the pens in the tool.