Two comments

1. It has been reported here that foam occasionally shrinks. Not much, but perhaps enough to pop glue joints as you are planning them.
2. Free standing foam is resonant enough to amplify train noise. It's not much with N gauge equipment, but some HO modelers find it objectionable. The resonance can be damped by bonding one side to thin plywood.

My suggestion is to glue your foam to 1/4 plywood with a foam rated caulk (spread thoroughly to eliminate voids). The resulting sandwich can be cut to shape nearly as easily as plain foam, and will securely hold items like switch machines and wiring blocks. Glue and screw two one inch strips of ply to the sandwich at each bracket to form a slot. That will keep the structure from sliding around. Drill a hole in the base plywood to capture the little nub at the end of each bracket. Once you have the bench exactly as you want it, caulk the brackets to the ply.

Please post photos of your progress.

pqe

Take care with the brackets

Take care with the brackets as far as front to back levelling is concerned They usually have a very slight slope towards the front.

 "glue your foam to 1/4

and I suggest making the foam only as thick as you need for scenic purposes and increasing the plywood to 1/2 inch if possible for better screw holding ability. 1/2 plywood over 1 by 4 box grid framing easily spans 36 inches on my N scale layout with no foam needed( modeling flat lands where elevated roadbed gives enough height for any scenic needs).....DaveB

Further thoughts

The easiest approach may be to work with the plywood first, getting it fitted in place, then bonding it to the foam and trimming the foam. If you need to splice two chunks of plywood, use a third piece underneath that runs from bracket to bracket.

There is nothing sad about doing a chainsaw. All layouts are chainsawed eventually. You do your best, with the understanding that you will get better, and what you do today might be redone tomorrow.

pqe

The "T" or "L" shape of the

The "T" or "L" shape of the girder will enable me to level each bracket and compensate for any slope. I'll try to take  picture of what I envisage, hopefully that will make more sense.

I was going to fit the foam first, cut it to shape, and then trace it out on the ply but I see the advantage of cutting only once.

I'd like to stay away from ply, but it might be a necessary evil. I don't plan on any switch machine, so its not a necessity but it is easier done earlier than later.

"2 foam should serve all landscape needs, there is a bridge to be placed somewhere on the track plan.

Some pictures

Here are some picture of a quickly made up T girder and bracket:

Hopefully this explains a bit better what I have in mind.

My shelf layout

If you haven't, you might look at my MRH blog. The first part details the construction of the benchwork/shelf. I don't think it generates any objectionable sounds. 

I have

And would have loved to use those double track brackets but even with my modest '26 or so possible mainline, the projected bill was over $300.

I am adapting the idea of someone else here, which at the moment I cannot remember nor quote.

The reason why I am considering gluing a "1/4 ply to the foam is that it will easily be screwed to the T girder bracket.

Now if I could just get off my lazy azz and install those shelf track on the walls....

Drain pipe?

What is the diameter of that drain pipe and is it going to be in your layout shelving?  There has been many a layout that had a drain pipe passing through it.  There have been just as many creative disguises for the pipe.  One seen often is to turn the pipe into a water tank at bench level with sky painted above it.  There have been town water towers too. Depending on your era you might have one of the more modern enclosed large diameter water towers with the town name painted on it.  How you decide to disguise it is up to your imagination.

Den

Intrusive enough to make it an annoyance

But the backdrop resting on the shelf track should hide it nicely: no such luck, it sticks out too much. Depending on real estate required for track, the backdrop may start curving before, but this is getting way ahead of myself. My concerns with the pipe was how I was going to notch the open frame. With no open frame, I just have to trace around and cut the foam.

Giving credit where credit is due