Raised on the Erie Lackawanna Mainline- Supt. of the Black River Transfer & Terminal R.R.
I think you have laid it the options and reasons pretty well. It's easier to cut the plywood while it's on the joists - more support under the plywood makes for lesss vibration and smoother cutting with the saber saw for somebody of limited skills like me.
But once the cutting is done, it's easier to build the scenery if the roadbed is up on risers. If risers are not universally used, the scenery has to come up to roadbed level at every joist, which is going to be a major obstacle to realistic terraforming. Risers and cleats are usually kept under the plywood, with the cleats only extending beyond where scenery support was specifically desired. By keeping risers and cleats under the plywood, scenery can drop to top of joists (or lower in between joists) whenever or wherever desired or needed. At least that's the way I did my cookie cutter.
An unmentioned advantage of having the plywood on risers is giving easily accessed space above the joists, but below the roadbed, to put the wiring and any mechanical linkages such as turnout throws. When I did lay my plywood directly on the joists, I ended up drill holes in the joists and running the wires through the holes. Even then, wires would occasionally get snagged while reaching or working under the layout.
Another thought: before you cut or adjust your legs downward, try the layout at the higher level. You may like it higher than originally planned. I did. But if you don't, lowering the layout with L girder framing is not all that difficult. I found L girder to be the best and most flexible benchwork solution provided benchwork "thickness" and weight would not be issues.
my thoughts and experiences, your choices
Fred W
As far as cutting the plywood, it is easy to place a couple of 2x4s along the plywood sides so that the saber saw avoids the framework. Risers can be easily added later to set the plywood on its permanent grades, and the risers will be located more accurately under the roadbed.
Regardless, the base level of the plywood should be at least an inch or two above the framework. This will allow for below-track scenery effects such as ditches, shallow gulches/streams, culverts, and little bridges. That will make your layout much more interesting.
Mark
I think that Linn was concerned about cutting the joists, or having the joists interfere with cutting, much more so than cutting the L girders. IIRC, he advocated 1 x 4 joists (gross overkill!!) which are 'way deeper than any saber saw blade.
The nice thing about using risers is the freedom to use anything for joists - mis-matched, odd-height and even split-top wood can be used. My own choice is steel, but that's a climate thing.
You might consider making your risers out of the smallest available steel stud material. It cuts easily, screws to risers and subgrade with really small screws, and screwing flanges for the roadbed can be snipped and bent in seconds (no separate cleats.) Try it, you may like it.
Chuck (modeling Central Japan in Septmeber, 1964)
Yes, it is perfectly OK to cut the plywood while it is fastened to the joists - as long as you move each one before you cut it. In fact, the plywood is typically better supported on the joists, making for less vibration and hassle while cutting the plywood.
(Quote) My joists can always be moved later if I should decide to run a creek or river, etc. below the zero elevation flat ply areas, yes? Thanks in advance for giving me a reality check to see if I'm understanding you correctly. Fred, I have my lower bench at 48" and my upper level (on future risers) will be at 56" just where I want it. If I need to lower the benchwork, I can do that by screwing the height adjustment bolts in 2" and then adding the 2" risers to raise the ply flat top. (End Quote)
I am concerned that you seem to be describing a second deck at 56" in the quoted portion. 8" separation between decks isn't enough. You can't reach into the lower deck easily, and you probably won't be able to see much of it, either. Planning multiple decks is fraught with compromises over access, overlaps, elevations, lighting, and viewing angles. Usually, one ends up at at least 12" rail to rail separation between decks, with 18" even better if the resulting deck heights are acceptable. Almost a mandatory part of multi-deck construction is minimizing the thickness of the upper deck benchwork - a situation for which unmodified L-girder is not well-suited.
OTOH, if you are talking about a track elevated 8", but on the same underlying benchwork (or deck), that's a different story. Whether track ranging from 48" to 56" is optimum for you is a personal decision. As you mentioned, the low/high points of the range are adjusted by where the legs attach to the girders/joists and/or by trimming or replacing the legs.
Whether the lowest elevation subroadbed is left directly on the joists, or is elevated by risers, is a personal decision. Leaving the lowest plywood on the joists will limit your terrain flexibility considerably when compared to putting the subroadbed on at least 2" risers. But if you can live with the limitations, there is no reason to move the plywood up.
Hope this makes sense and helps.
You could look into using Woodland Scenics subterrain / risers system. Be easier to raise the track a couple of inches rather than do all that cutting.