I didn't accurately represent ScreenSpaceLines3D when I called the class a kind of "line primitive."   Unlike a primitive, the class, when rendered, will create a line that has a constant thickness in screen-space.  In this way, it can be reliably seen regardless of its depth.  This constant thickness is a different effect than if you were to take a cube and scale it along a single axis.  The WPF engine is doing work to create this illusion.  Thus ScreenSpaceLines3D lines are also known as magic lines.  Check them out!