The top group of four photos is a 33 1/4" wide around the room module set actually being fabricated. 

When using 4' X 8' plywood, 33 1/4" wide modules may be one of the best sizes to maximize depth for modeling, economy of construction materials and practical reach for building and servicing your RR.

The second group of photos is for a 12" and 15" wide module mounted above an existing layout.

The valley loop:

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An overall view from the lift out rearward:

An overall view from the valley forward while laser leveling:

A link connecting two walls of modules:

Below is the second set of photos. 

The top pair of pictures has top and bottom views of basic modules as per norm. 

The middle pair show the overall of the oval & the extension's underside. 

The bottom pair of photos shows the modules with the requested installation of Ross and Atlas turnouts and track.

The wye is attached to the upper left corner of the oval in these comstruction photos, also a bottom shot of the passenger terminal & wye w/ three block panel:

 

The inner or outer loop trains can be turned complete on either the wye or the reverse loop via the Ross double crossover.

Note also, the raised side panel (left upper edge of photo) of the benchwork which maximizes duck under clearance.