Chaos you have to select COMPLEX at the top of the screen. That lets you hit the Forward and Back buttons on the right to move the grid forward and back. Move it until the riverbed anchor is white and then you can select it. To connect it to something in a different vertical plane you select it then move the grid until the "something" is white.
Or, you can build just the deck and drop a heavy steel beam straight down as far as it will go right in the middle. Test it and you get the wacky funhouse road. Alot of it breaks but it hangs together and the train can make it (not sure about the passengers though).