All of my structure is built in modules. Each module connects to the next using a nut and bolt.
I try to keep each section of structure approx. three feet in length. Any longer and the sections become more difficult to assemble.
The connection is made by having one of the cross girders be .040" styrene while the next cross girder is MDF. The L section with the end cross girder of .040" styrene "slots' in next to the girder made out of MDF. Before attaching the other girders, I drill a hole in the two girders. I can then insert a nut and bolt to connect the two sections.
The below photos illustrate this modular connection.
This is a connection between a two track straight section and a the begining of the island station. The track to the right curves to the right to clear the island station. The grey MDF cross girder can be seen along with the .040" cross girder of the unpainted section. A nut and bolt holds the two sections together.
This picture shows the reverse side of the above connection. The unpainted girders have been glued to the .040" styrene cross girder using super glue. Reinforcing angles will be added later. For the angled sections, .010" styrene will be cut into squares, bent to match the angle, and used as reinforcement.
Here is another picture of the connection. In this picture there are three sections of L structure. One in the foreground, one in the middle consisting of two girder spans and one in the background (the bolt is visible in the background).
I've found that this modular system works best as it allows the various sections to be built seperately and more easily. In addition, I can then remove sections from the layout as needed when working on the underlying scenery.
None of my structure is attached to my benchwork. The structure sits loosely on the "stone" foundations I make out of 1 / 4" MDF. Once the positions of the columns are determined, only those foundations are glued to the benchwork.