I have built four of these kits for my layout (two tri-levels, two bi-levels) and they are one of the easiest large kits I have ever assembled. I start by assembling all of the vertical supports, cementing them with 3M Super Glue Gel in small drops. After the vert supports are dry I then insert them into the slots on the flat car deck and glue them. Then I add my first row of cars on the flatcar deck (for large cars they loaded 4 per level, mid-sizes were 5 per level, and compacts 6 per level)- then I carefully glue the lower deck of the tri-level in place (or the top deck of the bi-level)- the lowest tab slot is the one you need to use for the middle tri-level deck. The middle tab is the bi-level’s deck tab and the top tab is for the top deck of the tri-level. I also add the safety rails to the middle deck of the car and the automobiles (it’s alot easier than cramming them in there after it’s glued) and secure them to the deck with small drops of Elmer’s White Glue. After that I add the top deck, scratchbuilt railings and cars. It should also be noted that there were versions of the bi-level Paragon (the rack type Accurail modeled) that did not use the X-brace either, check a photo reference if possible before installing the braces (on the PC rack I modeled- the prototype didn’t have the X-braces).
The cars that I use for my loads are the Fresh Cherries HO scale die-cast cars- contrary to popular belief- these cars do not make the rack too heavy- nor do they make the car top heavy when loaded. The car will weigh about 13.5 ounces loaded with 18 cars- while the empty rack weighs in at a spindly 6 ounces. My bi-level is loaded with the Malibu Reel Rides die-cast Chevy trucks- same story there. For less than 2.50 apiece these cars are well worth it for your loads.