For my next laout I am planning an around-the-room shelf layout. Unfortunately, it will pass about 4 to 5 inches below some 2-foot wide windows. My current layout passes under a 6-foot wide window which leaves quite a gap in my 18" high backdrop. I thought of just continuing the backdrop past the window or boarding up half the window. Probably look a little tacky. I even saw a layout with the backdrop painted on pull-down roll-up window shades, but that still looked sort of “artificial”. Any other suggestions?
I have not done this so this is pure brainstorming speculation. What about using various foam sheets to create a sort of negative image of the window, by which I mean sort of like foam packing pieces which fit tightly over the television or stereo or camera or other enclosed item. It could press fit into place (and thus be removable if need be), with a perfectly flat surface facing the outside world, painted blue to match your backdrop. It would be lightweight and easy to handle, should be fairly easy to construct. Yeah it won’t create a seamless blend with the rest of the backdrop but I have a feeling nothing ever would.
Dave Nelson
If the room is a write off for anything but trains who cares about the window. I am usually in there at night, so don’t even notice the window. In the prebuild stage I thought a lot about the window, but once started I had completely forgotten about it.
Brent
Here is my suggestion:
Put a shutter blind on the window, so it will still look nice when viewed from the outside. Make the part spanning the window removable, so you can still have access to that window - even though the room is reserved as a train room, you may still want to be able to open that window now and then.
yeah shutter type shades might work better you could paint the one side to match the back drop or even put a print of some trees on the shutters
As an architect, occasionally we have faced the need to blank out an existing window to deal with interior conditions. What we’ve done in such cases is to take a panel of plywood, painted black on one side, and tack it up to the window frame such that through the window, you just see black. You could do the same here, then run your backdrop right in front as in the photo by Batman. That’s assuming you don’t need the light from the window at times, of course.
If you do still want the light, then you could do as Batman did, but just paint the back side of your backdrop black. You have to do that painting prior to installing the backdrop, naturally!
My HO around the room layout is in a garage loft, with single 24" widows at each end. I use SceniKing sectional continuous panorama with uniform blue tops. I painted the sky to the ceiling with electronically matched blue paint. I removed the casement around the windows, so that the windows are flat with the wall. I placed a pull-down shade in the window opening, from ceiling to the bottom of the window, so that people on the outside see the shade, while the inside is a continuous panorama. The photo shows only a portion of the entire covered window scene. The shade can be rolled up, as needed. The arched truss bridge helps hide any visual distractions, of the transition between the panorama and the cut-off photo of a waterfall below. Bob Hahn 
I like the looks of Batman’s solution, but would paint the walls sky blue.
Be sure to make the backdrop across the window either easily removeable or easy to break through. The window is probably considered to be a fire exit, so making it totally impassable would probably be a violation of code - not to mention the inadvisability of becoming cannibal bar-b-Q in the event of a fire in the hallway.
Chuck (Modeling Central Japan in September, 1964 - across a garage door that can still be opened in emergencies)
I have been thinking of using pull-down shades over my windows and continuing the backdrop right over the shade.