First off, if you haven't taken a look at our Layout Design forum I suggest you do so and read through several of the threads. Planning isn't scale-specific as far as concepts are concerned. Secondly, I wouldn't consider your layout "small" but it certainly isn't a large layout. An 18' run is a lot of space. Now, you indicate you want coal and potentially milk. This points toward a steam or transition-era layout, which generally means shorter freight cars and locomotives. Now the next question is how big of an operation are you modeling? Are you modeling a small part of a larger railroad, or a smaller railroad that interchanges with a larger one? What is the geographical area of your railroad? This will have a bearing on what motive power you run and how your interchanges could be handled. Next, think about your operation. You have stated you want mainline run and a coal mine. What else do you want to do? Do you want to switch a yard, run locals that service industries, or just keep it simple with a coal mine? What about your milk operation? I have experience with milk operations on my club layout and can offer some suggestions if you'd like. Your coal operation could revolve around a single mine. The easiest way to have someplace for it to go is to run it "off-line" to a staging location. This could be as simple as 2-3 tracks that are about as long as your longest train (15-20 cars or whatever you decide). Loads out of the mine, run around the layout, then off to staging - empties would be the reverse. When you are done operating you can reverse the process or do it manually. Alternately you could include a yard on-line that would serve as an intermediate staging location between these two that would be a crew/power change location. Your mainline will probably be a simple dogbone or folded dogbone style one. You can run it as "single track" but bring the straightaway together for the appearance of a double-track railroad. These are some suggestions and questions to get you going. Keep asking yourself what you want in your layout - when you can answer those questions you can proceed to actual track planning. As you come up with issues when you plan you keep asking the questions over and over, and finally you'll have a finished product.