I remember playing through the first Cargo Bridge, and getting so frustrated I quit. It's rare that I quit a game because I find it too difficult. My problem was that I expected the physics of the game to work in a completely different manner than they do. Adding trusses and supports to your bridges in most cases simply adds weight to the bridge instead of stability. After observing the physics from the first one, and adjusting my strategy accordingly I managed to beat several of the levels in the Xmas Pack on my first try.
I'd like to point out that all of these bridges may not work for you even though I tested them all multiple times. I've ran into situations where I make a successful bridge, then replay the same bridge only to have it fail horribly. Additionally you may have to make a few adjustments to your bridges since you can't really copy these exactly.