I used to live in Denver many years ago.
You would be surprised at the different plants that can be made to look like trees and shrubs. Winter may not be the season to go out looking though, if there is a lot of snow on the ground. Find a field somewhere, or even along the side of the road, as long as you can safely leave your car, and start looking.
Many plants have branch structures that look like some kind of tree, the trick is to use your imagination, and then experiment a little.
By the way, when it comes to trains, you should really post your questions over on the Model Railroader forum, that's where the N scalers hang out, and you will get more and better responses. This board is where mainly O gauge trains are discussed.