The bridge assembly kit in the beginning was very easy to figure out, especially with the instructions given. The grooves in each piece made it easy to identify where the big pieces should be placed. However, once you reach the walkway, things aren't as obvious. The instructions say to push the bridge base in with a pencil, but without any grooves similar to the legs, it just falls right through the structure, not to mention the instructions don't show any motion to slide the piece anywhere. Same goes for the smaller pieces like the signal lights, which beyond placing the lights in the groove, it doesn't show how exactly the piece snaps onto the bridge. If there was a Youtube demonstration, that would make things easier.
Here's a Youtube video for a signal bridge but he doesn't go into much detail about the assembly.