We made our own modular shelves. The shelf units are made from 1” x 6” and 1” x 4” for the two sides (LH & RH). We made this as a box frame, with 1” x 4” for the two sides (LH & RH) and the shelf units are made from 1” x 6”. I use 1 x 6 cut to 3 ½ “ long and screwed to the sides to provide a spacer for the shelf. The shelf themselves are 1 x 6. This will allow 2 rows of O gauge cars on them. I make mine 4 feet long, this is common shelving wood at many home improvement stores. I make on set at 4 feet high and a second one at 2 foot high. Added together it makes a 6 foot high shelf, which clears most ceilings.