Hi Chris,
No expert on this by any means!
Have you checked out clubs on line to see if any pictures are posted showing there track for ideas
So please take this with a large pinch of salt!
I would suggest that if you can go for a raised track, 3 1/2" isn't wide & when you are sitting behind the center of gravity on a ground level track is all wrong! Had an interesting experience on ground level 5" when I was leaning out to check injector preformance as I started going around a curve!
So a couple of options would be.....either a cast "low" wall say 5 or 6" wide, say 12 to 18" high, not having your legs bent to strange angles helps the fun! Replace the concrete with blocks, don't think bricks would work. My local track has low pillars at about 4 or 5' at a guess with a concrete beam cast on top. Another track I visit used to have concrete blocks on end set on top of 400 or 500 square pavers, the track bolted to these, the rail was probably 30 to 40 mm hi & over time had deformed enough that as you ran you could feel the undulation.
I'm sure for a home track timber piles & beams would work fine. You could always cast pipe into concrete in the ground as posts with a tee on top & timber beam. The same could be use at a guess for the removable section, the concrete in the ground with a section of tube cast in & the tee section with pipe attached that would slip in.
Hope some of this may help.
Cheers Kerrin