So, you have created the perfect model railroad, including a fully landscaped environment, and you want to take it a step further and create a visual model train backdrop to complete the whole scene. There are many different ways to approach this.
One of the simplest model train backdrops is a basic blue sky background. With a tube of sky blue paint, and a little white for blending, you can create a simple yet effective sky backdrop. Paint the whole background blue, and slowly blend in white about midway down to the bottom of your backdrop, without going completely white. Streaks of white softly blended in can create the illusion of wispy clouds. If you are little more artistic, try sponging or stippling on clouds and adding a little more depth with light gray at the bottom of cloud formations. Use a picture as reference for the type of clouds you desire.
Another option is to use photographic posters or wallpapers as ready-made backdrop choices cut to size. Take it a step further, and create your own individual photographic background. Pick your venue, set up your camera on a tripod, and slowly snap away shifting the camera from left to right capturing a full landscape. “Stitch” the photos together with a photo-editing program and print out in sections. Glue onto board, and stitch back together again to bring your photographed landscape to life behind your model railroad.