By the way, If you are building a modular or transportable model train layout then there is no choice, you have to build each section of benchwork to NMRA standards so that they are compatible to all other modular sections you will encounter when you transport and show your model train layout.
I have had model train layouts of all types. From the loop around the Xmas tree, to the 4'x6' Murphy bed type, to the couple of 4'x8's slapped together, to the transportable modular, to the typical sectional benchwork kind. They all served a purpose, they all ran trains, they all taught me something.
|4'x8' model train layout|
Benchwork needs a bit more planning. A detailed track plan should also be in hand before designing your benchwork. Hey you need to know which way the rails are going first. Model train layout height and depth need to be considered before building. Realize the limitation of your reach no more than 36" deep and keep the height around 42". Model train layouts look the best closer to eye level but are harder to work on the higher they get. Make the sections of your benchwork a reasonable size 4' or 8' long since you are probably buying 8' lumber and you may have to move one day. Plan on making frames of 1x4 with cross supports about every 12". For legs you can take two pieces of 1x4 or 1x3 and make L shape beams to transfer the weight of your model train layout to the floor. Then just screw or bolt the sections together in the shape/pattern you need for your overall plan. Using benchwork your track plan and layout shape doesn't need to be a square or rectangle, it can have all sorts of shapes to it. Quick Tip - Predrilling your holes with a pilot bit will stop the 1x3,4's from splitting.
Benchwork has an open frame which can allow you to put a flat surface on it when needed or easily attach vertical "T"supports for scenery or for subroadbed. A lot of guys are using extruded foam insulation sheets to handle all the subroadbed as well as its supports, but plywood is the old standby. It's all preference but those are the two main different mindsets of building subroadbed.. Using the open grid work style benchwork you have the most options and flexibility for your model train layout.