Updated 7/1/2019:
Started on the benchwork east of the Stone Arch.
First, I put another cross-member into the lower table (note the corner reinforcement).
Next, I temporarily clamped an upright to a cross-member to support the sub-roadbed. The sub-roadbed will receive a framework of 2"x2"s underneath it for support.
Then I cut a board that will serve as the eastern-most abutment. The Stone Arch will be screwed to it to anchor that end of the span. The board will be fastened to the plywood below it as well as the sub-roadbed itself.
Here's a side view showing the different heights involved. Track on the Stone Arch will rest upon doubled 1/8" black Woodland Scenics foam roadbed. To the right, it will abut the 3/8" Vinylbed atop the plywood sub-roadbed.
More when I know it.
George