Bench Support - Model Railroader Magazine

I didn't read your entire OP.  I missed the part about the 1/4" plywood.  To give you some reasons for my recommendations, common goals for benchwork:

• minimal deflection under expected loads.  Expected loads may or may not include leaning or falling against or climbing on the benchwork.  Linn Westcott's book on benchwork has tables for spans using various wood structures that assume maximum allowable deflection of 1/500 inch with a 250lb point load.  You can't allow the same deflections that your house floors are allowed because any significant deflection means derailments.  Whether you allow for 250lb loads is a personal risk decision.  House floors are 3/4" ply on 16" centers.
• stability against sagging, warping, or other dimensional changes over time.
• easy to modify for changes.  This usually implies all screws driven from below, use of screws instead of nails, ability to relocate/reposition parts of benchwork without taking benchwork or the layout apart.  It also implies that the benchwork is sturdy enough to remove a joist or crosspiece without losing its structural integrity.
• light and thin enough to easily remove sections from layout room for moving or taking to show, etc.

In your case, the last apparently does not apply - I doubt you would use 2x3 studs if that were the case.  I'm not sure how much priority you are giving to being able to modify easily.  To me that's a big deal; to others not so much.

I'm assuming from your description and dimensions that you are using a ladder style grid that is 2-3 ft wide, using 2x3s for both the rail and the crosspieces (joists in Westcott's book).  I am also assuming that your intention is to fasten the 1/4" plywood directly to the rails and joists from above.  If this is true, please don't.

1/4" plywood is quite flexible.  When laid flat, it needs to be supported every 8-12 inches to prevent it sagging between supports.  And, because of its flexibility, it will conform itself to any uneveness in your support structures.  Did you true up the top surfaces of the rails and crosspieces so that they are all within 1/16" of each other in the same horizontal plane?  And is the wood seasoned enough that it will remain within 1/16" over time?

Westcott recommended 1/2" plywood on supports space at 16".  I have used 1/2" plywood at up to 18" support spacing successfully.  But my layout was full of grades, so that the plywood was generally bent.  Bending plywood stresses it, and makes it much more rigid than laying it flat.  I should note, however, that this was in the days of much better quality 1/2" plywood.  I used 5 ply AB grade.  And there are several known examples and reports where over time the 1/2" plywood sagged between the supports but the Homasote on top did not.  That's why Lance and others with experience recommended 3/4" plywood.  The other reason for 3/4" plywood is that will not conform itself readily to small uneveness in the tops of the rails and joists.  Instead, it will bridge the low points.

The second point is that using risers and cleats to raise the plywood up above the tops of the rails and joists has several advantages.

• the space between the plywood and framework is available for below grade scenery
• the tops of the joists and rails do not have to be trued up.  The truing up takes place by adjusting riser height, which is much easier than planing or sanding benchwork grid tops.
• If the joists warp or sag, the risers can be readjusted to true up the subroadbed again.
• all screws can be driven from below.

The disadvantage of cleats and risers is the extra work, and it makes the whole assembly "thicker".

Other possibilites instead of getting thicker plywood are gluing foam or another piece of 1/4" plywood to your existing 1/4" plywood.

I have used both 1.5" and 2" extrude foam glued to 1/4" plywood very successfully.  The result is rigid enough to space supports at 24".  Just be careful to glue the 2 together on a totally flat surface.  Otherwise, the combination will pick up and hold forever any curves in support while the glue sets.  Don't ask me how I know this.

Gluing a second piece of 1/4" plywood also works well.  The result is stronger than 1/2" plywood because of fewer voids and more plys in the sandwich.

just my thoughts and experiences

Fred W