Mike,
I assumed that the "baseboard" was the room dimensions. I use a polygon to trace the shape of the room (made by using absolute points for a baseboard, like you did).
One can now use the properties of that polygon to make the walls any height that wish.
I also use a polygon to create the decks or baseboards of multilevel layouts, or at the least the second and third level.
The trick to this is to set the height of the polygon minus the thickness of the material, then set the thickness.
The second trick is to make the track at that level set to the total height, and also select bridge. Then select all track and with height on, right-click and select remove track supports.
The result is that the 3D mode then looks like what you see in you train room.
I'll fiddle with the stairs/bath area, as built, for an example and post the file for you.
