Paint the rail. A simple and effective method of adding a bit of "pop" to the wall is to ignore the wall altogether and instead paint the rail in an accent color that complements the main wall color.
Incorporate the rail into your design scheme. If you are particularly artistic, you can paint in a variety of designs just above the rail that make use of the rail's height and appearance. For example, painting a row of potted flowers above the rail can give the unique image of the pots sitting on a surface. Similar designs can make further use of the rail within your design.
Use the dividing effect. Since the rail effectively divides the wall down the middle, take advantage of that when painting or decorating the wall. Paint the wall above the rail a different color from the wall below the rail to create a complementary two-tone paint job.
Paint everything -- the wall and the chair rail -- the same color. This effectively negates any dividing effect the rail has on the wall and gives a basic monochrome appearance to the room, if that is what you are looking for.