Picture of Double function rail: hang paintings and indirect lighting
This wooden rail has two functions:

 1. Allow to hang paintings without the need of hammering a nail into the wall; and
 2. Provide light indirectly illuminating the ceiling: atmospheric lighting.

The rail has been fixed to the wall at approximately 50 cm (20 in) from the ceiling. With a thin and colorless cord paintings can be hung easily from it, without the hassle of nailing every painting. Moreover, it allows to adjust the location of the paintings once they have been attached.

The indirect light is being provided by a rope light, which has been countersunk in a slit. The depth of the groove is enough to make the rope itself invisible when looking at it from the room in a standing position. In order to reduce electricity consumption, preferably use an LED rope or an LED strip. Using LED is safer, since it doesn't heat as much as conventional filament rope light (see Step 1: Safety and risks).

The rail described in this instructable is the second product released by openproducts that involves indirect lighting, and considerably less work than the Giant Ceiling Light with Multiple Functionality a.k.a. the UFO, which has been released (under CC-BY) on September 13th 2012: http://www.instructables.com/id/Giant-ceiling-light-with-multiple-functionality-a.

The design, the making, and the installation of this rail light has taken a few hours. The effect is fabulous. 

The next steps discuss safety and risk issues (Step 1), detailed design features of the wooden rail (Step 2), tying a cord to the rail (easy way, Step 3), tying a cord to the rail (more elaborate way, Step 4) and licensing (Step 5).