That's from a Kalmbach book, I'm pretty sure (I can picture the article--pink on B&W print--I know I still have it), and it's wonderful work you've done here--huge applause.You're going at it in the wrong frame of mind RE.
The build is part of the pleasure of modeling. That's why they call modeling in the present tense.
Now is the time to ensure the bridge is constructed properly. It's hard to fix deficiencies after it's built and scenery is in place.
I went through the struggle of how much is enough when I was building my Canyon Diablo bridge.
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It's very good foam carving, BTW. That said, and if you aren't already aware of it/them, there are also hot-wire knives, which can chew through multiple layers of stacked foam as though they were one solid piece (without any layers at all). I do know there are several Youtube videos on the subject, and I'm pretty sure there are some DIY/build your own handheld (as with an extended length, flex-shaft Dremel tool), versions. Luke Towan has built one I know, although his might be more like a desktop bandsaw than a handheld carving tool. I do know there are some others too.