Jeff, far be it from me to presume that I can speak for Tod but I believe he's referring to making something similiar to a box beam for rails instead of just a single thickness member the rail.
You take your rail and add a top, bottom and another side to it to make a box or a square tube, thus making it thicker, adding 3" to each side.
I'm sure that you will get a better explanation but I wanted to try to explain quickly before you do anything drastic.
Maybe someone can do a quick drawing to explain better.
My diet is working good. I'm down to needing just one chair now.
"Just think how stupid the average person is, and then realize that half of them are even stupider!" --George Carlin