OK, givens and druthers...
The layout area is as shown above. The layout will probably run around the permiter of the available space, to make the best use of the area and also allow acces. A peninsula or similiar feature is also a possibility though.
Duck-unders are not an issue, and in fact I fully expect one.
Layout will most likely be single level due to storage areas under the layout. The track is already 54" high, adding lower level staging, which would still have to be at 54", will make the main layout too high above the floor.
I may consider building an upper level on a portion of the layout for a mine or other scene. I don't have the time to build a complex mushroom style multi level layout though.
Steam/Diesel transition area
rolling hills/eastern mountains
turntable and roundhouse, with 120' turntable. (already on hand, must be incorporated somewhere...)
primarily designed for continous run, preferably with at least some alternate routes.
double track for at least a portion of the run, both to allow simple multi train operation and to give a PRR style feel.
The track on the brown area is limited to the corridor shown (Shelf supports obstruct the white areas) It is permissible to have restricted access to that track, it can be blocked by benchwork in front of it for example.
Good access to all areas is nice, but not required. For example some of the corner areas may be hard to get to, that's OK.
#6 turnouts main tracks, #8 in crossovers and wherever possible. #4 in yards are OK.
2 percent grades preferable, 3 percent max if possible.
20 inch min radius on main tracks, 18 inch yards, sidings, curves.
Easements where possible
Bridges and tunnels if possible.
City scenery in station area, but only enough to suggest large town.
Wooded hillside in at least one location
small town/mill town feel, possibly blending to station city.
I'd like a small switching yard (3 to 5 tracks or so)
At least a few industries would be nice, in keeping with the PRR theme coal mines are a likely choice.
A mill scene would be great but space may preclude it.