Updated 10/8/2017:
The photographer will be here tomorrow to inspect the train room and get an understanding of size and position for the backdrop. To prepare, I took a laser level and established the 50" layout height all around the train room. I also placed the bridge in its approximate location and orientation. From west to east (left to right):
The red laser line is visible in this photo. I have used yellow post-it notes to indicate the height on the walls.
The brown paper wrapped object is the bridge. I'm keeping it in its protective cocoon.
The bins at the far end of the room denote the position of the backdrop.
George