How do I attached 8' long wooden flower boxes to a wrought iron railing?

I am assuming that you are trying to attach them to the side of the railing and that you do have posts going up to the railing that maybe you could attach them to. The first thing that comes to my mind would be to try using car muffler clamps or any round type clamps. Would need to drill a couple of holes in the side of the box to accommodate each clamp. This would probably work if the boxes are not going to be too heavy.

If the boxes are going to be heavy, then I would try to using ornamental or plain shelf supports under the boxes.

You might also find some helpful information concerning attaching on the following sitie.

How To Buy Or Build Window Flower Boxes

I would suggest galvanized "U" clamps every two feet, set on the upper half of the back of the boxes (you can find them either in the hardware or fence departments of your local hardware store).