Basic Idea for the old main of the B&O include the Carrollton Viaduct

Where on the plan is the viaduct located?  How would the owner access the room?  There is a door in the upper left, is a duck under the option considered?  Is this going to be an operational oriented layout, ie local switching?  If not and it were me I would want another return loop in the upper right or a pass through the storage area with a second return loop either below or above the return loop in the lower right.  Or just a link to that loop to save space that permits continuous running.  Great use of space however, I like it.

Location of viaduct will be along 1 of the 2 center bubbles--yet to be determined.  A work-in-progress.

Access is easy!  The photo is terrible.  There is a stairway into the basement at the right edge of the picture just above the storage room.  You simple walk into the layout area.  No obstacles!  There are no duck-unders!  The door leads to the outside.  There will be a hinged section across the doorway for access/egress when layout is not in use.

Layout will be for operations.  This is the first basic sketch as the owner requested 96 diameter curves and I added the four foot minimum aisle ways to ensure people may easily pass one another as they follow their trains.  There will be switching areas and also spurs along the route.  There will be at least two passing sidings.

There will be a provision for a continuous run made--it was not added as to not "confuse" the drawing.  On a CAD program I am able to turn on or off layers.  For a photo, you have to pick something.  I posted the very basic route.  In the old days this would have been called an out and back.