I have a Creative Playthings playset with a 9' swingrail. While the playset is wonderful in many respects, it suffers from the fatal flaw of just not having enough of the right kind of triangles in the structure.
Kids get on the swings that are hung from a 9' swingrail and the lever- arms against the structure are huge. Thing skews back and forth in a terrifyingly swaying manner no matter HOW much the bolts are tightened.
I put a big 2x6 overkill diagonal on the thing and it doesn't budge a millimeter.
Here's the picture. I've kept it at original resolution so that you can see the bolt placement best.
* The 2x6 is from Creative. Their engineers had to approve my design. I just couldn't find 2x6 cedar anywhere near my house. * The top and bottom bolts are reusing the existing T-nuts (and bolt holes) that were there. * I have engineering OCD so everything within 500' of a drop of water is stainless. That added a hugely unnecessary cost to this thing. Just use grade-2 steel. All 5/16ths * The middle bolts weren't perfectly placed but I was starting to wear out mentally.
Hope this helps someone. Truly though, a 2x3 stretched from the topmost left to the bottommost right with a single bolt at each end probably would have been ok. I was just worried about pushing strength.
When properly trimmed and painted it loses that "Nazi Watchtower" look. I think it's ok now.