I have the kit. I believe it was everything to make the bridge wider for double tracks. So basically nothing is double supplied. The cross floor supports are double wide and there's the second deck work. I believe the double wide upper truss work is there too. It takes the single track bridge to make a double track. The sides aren't included.
I have it downstairs and it has sat there for a few years now. I'm finally just starting to build my O scale layout!
Let me know if I can help with exactly what else you want to know. I'll take a picture too if that helps.