Josh,
I would recommend that you just submit part one as a track plan concept. I suspect that if you submit it as a possible 4 part article with parts 2,3,&4 being done as you get modules built, you can expect them to probably reject it without looking at it. The only way to get the 4 part series looked at or published is to submit all 4 parts completed at once. As was mentioned earlier, what they publish is determined by what they have decided they want to cover in the future. The magazines also pay $300.00 for each article published (at least they did 5 years ago, it may be more now). They won't pay for anything less than professional quality work. Make sure your spelling, grammar, and punctuation is perfect. Needless to say you will need to get the names of all of the industries you want to model, and it might be worthwhile to make a trip to Modesto to get photographs of the industries and locomotives and rolling stock. Not only will such a trip give you valuable insights on what to model and how to do it, but it will also give the magazine some pictures to go with the article. As you build the modules, you can photograph them and write future articles to submit. You might get the original article published in MR with the articles on building the modules published in RMC. Just remember that the modules and their photography as well as accompanying text must also be of professional quality standards. If you look at the photographs of model railroads in the magazines, you will notice that the pictures show either dedicated layout rooms, or the shots are so tight that nothing extraneous shows up in the picture. You won't see a window behind a scene, or a post holding up the roof of your house, or anything else that is out of scale in the pictures. When dioramas are shot out doors for "Trackside Photos", notice that any background trees are distant enough to look like background to the layout, not out of scale. Unless you are doing a scene where track workers are in the process of rerailing a locomotive or car, all of the wheels should be on the track. Photographs are very unforgiving. We may overlook flaws with the naked eye, but the camera will reveal every single flaw in a scene. The Gauge community is very forgiving of one another's pictures. We enjoy seeing the layouts that others are building, and if there are out of scale things like an air conditioner in a window, somebody's bedroom furniture, or other distractions in the pictures, we overlook it and enjoy the model railroad that someone is sharing with us. The magazines will not be forgiving. They want everything perfect for publication. Look at the pictures on the Gauge of scenes from Dr Wayne's layout. Go to the Academy and look at the pictures in "Shamus Narrow Gauge World". We may think those are outstanding pictures, but to the magazines they are not. For publication that sort of quality is the minimum required by the magazines.