A model railroad is like any other creative effort: it requires a vision of what it will be when completed. You have to have a concept of what your model is representing, and without this vision, you will not have a blueprint for what your layout will become, and will most likely be unsatisfied with the result.
To give an example of what I mean by a concept of a model railroad, I will use mine as an example. My layout is a fictitious representation of the Adena, New Athens, and Cadiz Railroad. This was a coal branch in the Eastern Ohio Appalachians, jointly owned and operated by the Pennsylvania Railroad and Nickel Plate Road. It is roughly based in 1956, so I run transition-era equipment. On the layout is a coal yard with a small engine terminal and a large coal loader. The goal is to serve the coal loader and also sort and dispatch mine runs from off-layout locations. Since this is a remote mountain area, there are more trees than anything else, and the trains are dwarfed by the mountain scenery.
Of course, I have a ways to go on completing this vision, but I still have it. Without the vision, I wouldn’t know what to do with the scenery, I wouldn’t know what locomotives and cars to buy, I wouldn’t have an operating scheme, I wouldn’t have much. The concept gives me not only a guide, but goals and a motivation to keep working until the concept becomes reality. That is why I highly recommend that every project you start should have a clearly defined concept. It makes the process go smoother and easier than it would without it.