Sagging Deck rail repair
Since I was the commenter in question ;-)

Doubled wedges are just wedges put point-to-point and driven towards each other - the wedge is one of the basic machines if your history class even touched on those.

I'll be very amused to see alfreema try what alfreema has written. The wedges described there are at a very steep angle and would be quite difficult to make work in practice. Something more like a 1:12 angle works a lot better - if you have a 4 inch gap you need to be a 5 inch gap, you put 3 inches of blocking flat, and drive two 1.5" thick (at the fat end) 12-18" long wedges point to point on top of the blocks.



If making the wedges daunts you you can buy plastic wedges for felling trees at a logging supply, or you can use cedar shingles or shims, which are sawn with a taper (they are rather soft for some heavy jobs, but would be up to the railing job) - you may need to use a sizable stack of those since each one is quite slim - but you can do that - and they are 16-18" long with less than 1/2" of taper, so the wedge angle is low (which makes it powerful - at least until the force is so great the wood crushes, where a harder wedge would be better.)