I couldn't see the forest due to the trees

How does that old saying go? "One step forward, two steps back."

That is exactly how I'm feeling right now, but I think it will be worth the trouble in the end. After painting the trees at the entrance of the yard, I let them sit for a few days to see if I liked the look. While I was happy with what I had created, something was still bothering me. A few nights later, that "something" finally revealed itself: I wasn't thinking far enough ahead.

When I painted trees on the backdrop around the main layout, I did it so that when I eventually "planted" 3D trees, you wouldn't be able to see a solid blue background through the branches.  

In the following images, you can see that after painting the background trees, I finished the basic ground cover. The actual scenery base comes next (I haven't finished it yet).

 

I did manage to get some trees made and planted behind the Ruston depot scene. As you can see, looking through the 3D trees doesn't reveal a solid blue sky all the way to the ground. In my opinion, this gives the scene much more depth.

So, I decided to continue painting trees across most of the yard’s backdrop, just as I had elsewhere. My plan was to stop the painted trees, add an AI-rendered building at the end, and then start a line of AI-rendered and printed trees. These were meant to extend all the way around the yard's backdrop, with a few breaks here and there.

My thought was to place a printed building partially over the painted trees and partially behind the printed trees, which hopefully would create a forced perspective and add some depth. However, after looking at the building flanked by painted trees on the left and printed trees on the right for several nights, I realized that was exactly what was bothering me. I removed the building before taking this photo, but you can still get the idea.  There was just too much of a difference.

The main issue here was space. Unlike the other parts of the layout where I could plant 3D trees in front of the painted ones, the yard has no room between the rearmost track and the backdrop for physical trees.

This is why I was looking into printing photos of trees and buildings in the first place—I just simply got ahead of myself. I realized I needed to remove the painted trees at the yard entrance. I tried painting over just the trees, but since some of the clouds were behind them, I decided to bite the bullet and entirely repaint that section of the backdrop.

A couple of nights and two coats of blue paint later, the deed was done. Of course, I then spent another night repainting the clouds, but they turned out better—a bit more transparent and fluffier than before.

With that settled and behind me, I focused on the AI tree line. I've played around with 2D drawings and photos before, but I'd never attempted to use them for a backdrop. The idea for the tree line actually came to me while I was experimenting with AI to create backdrop buildings. At one point, I wanted trees in front of the buildings, and I really liked what Gemini produced, even if it was just a visual experiment at the time.  Mind you, this is 100% AI

So I fired up Gemini again and started playing with the command prompts, which really are the key to AI. I asked it to render a tree line for a model train layout and had it add a solid blue sky with no gradient, adjusting the shade several times until it looked right. After tweaking a few specific keywords, it gave me this:

Once I converted it to a JPG, I used an eraser editing tool to remove almost all the blue sky so I wouldn't waste ink during printing.

Unfortunately, I didn't take pictures in the proper sequence because I was so focused on the task at hand. But if you look closely, you can see the printed blue sky is slightly darker than the painted backdrop.

While pondering how to remedy this, I continued to search YouTube videos and found a modeler who had already solved the problem.  Basically, after the glued images were dry, he went back with the same backdrop paint and carefully blended it onto the edges of the images. Voila! Simple and effective.  If you look closely you can see the printed sky in the first image, in the second image the transition is barely noticeable.