I haven't made any headway on the buildings, but I did manage to redo the turntable and clean up the main level decking a bit by squaring corners, etc. I also fiddled a bit with sample bench work both of which are on separate layers.
Since SCARM doesn't have the 32" turntable in its library, I mimicked it using a 32" circle with a 28" turntable inside. The Red tracks you see around the turntable represent the 2" difference between the 28" and 32" sizes. I needed them so I could position the lead and whisker tracks correctly. If you look closely, you can see how the bump-out is slightly narrower and the roundhouse is further from the edge of the peninsula. The roundhouse had to be moved closer to the turntable or the ends of the whisker tracks would have hit each other. Obviously, if you don't like the squared off benchwork shown here, you can always overhang the bench work framing and use a jigsaw to round it off.
Hers'a basic idea for bench work framing. It can be done with 1x3's, 1x4's, 2x4's or plywood ripped into 3" wide strips. This sample has uses 24" centers in most places, but don't show the extra support that will probably be needed for the turntable.
Here's a very crude sample for L-girder. The biggest disadvantage is the extra layer with the stringers on top of the rails adding 3"-4" beneath the layout. As you can see, I don't really know how to picture L-girder for this type of layout. I oriented the rails left to right and the stringers top to bottom, but I have no idea if that's the correct way and I wasn't sure about the lower right. I also have the rails on the edge of the decking and I believe they should be inset 6" or so. With the top and bottom being 24" wide, it just seems silly to have rails only 12' apart. The place where L-girder is a big advantage is on the peninsula and the decking below the upper reversing loops. You simply extend the length of the stringers to support the extra wide decking, but I've never built L-girder, so I have no practical experience.