Marx/Lionel O-27 Track Roadbed Scribe

I made a little roadbed scribe that I can use to mark the edges of where my O-27 ballast spreader (http://www.thingiverse.com/thing:1886250) should deposit ballast.

Easily slides over the rails while holding two regular sized pens or pencils on each side, marking a 12' O scale road bed with the track centered.