My modules are generally 16" wide leaving about 12" between the 2x3 legs. I got a 1x12 (11.25") 6' board to run between the leg supports of a center module and mounted a 1x3 down the center of it. It serves a two-fold purpose: 1) to line up the train boxes and 2) to provide L girder support so the board will not sag.
Stacked 3 high, this holds 135 car boxes on each side for a capacity of 270 MicroTrain boxes. Stacked 4 high would hold 360 containers. I have 1x6 single sided shelves along the wall modules for additional storage.
There are also wood-grained metal bookshelves under the wall modules. Houses, tools etc go in clear, stackable, plastic boxes on them.