There are many planning models to railroads which can be a hobby onto itself which can get beyond the design stage. Try to not be there to let your plans evolve your layout progress which will allow you to get everything right. Here are some of the things you need to consider when designing the model railroad and planning for any model railroads, which involves five steps.
Defining the constraints and basic goals which can allow one to have a better shape which includes the prototype, locale, era. It also has many operations which can be replicated, allowing one to have freedom of choice which is less essential. A model railroad does not have to follow a specific prototype which involves a lot of freelances, picking and choosing the right prototype which people have created on their own railroad names and schemes.
Space is an important factor which needs to be less fixed from the start, which covers a small bookshelf which can convince that model railroad with a bit creativity make it better for the whole experience. There are multi-level design, modular layouts and other creative alternatives which helps one to squeeze a lot of odd-sized shapes into spaces.
There is a lost of constrains from the minimum radius to maximum grade. There is no simple answer which does not follow an acceptable standard. The theme of your layout has a lot to say, which requires a few standards to be applied. Try to make sure that you have 18-inch radius curve which can make a standard train set curve in HO. Most trains run out better on car batteries which is not the best option for modern heavy mainline. It will work fine for HO scale which works on a narrow industrial gauge. There is an 18 inches N scale which is very comfortable for the curve. There is a larger scale switches to help finish out the layout which without any deep curves can have a lot of slow-moving action.
Time and budget
Time and budget constraints are some of the ways to be more realistic, which will allow one to afford to build and maintain. There are many modellers who use the home filled with unassembled kits which can call it home. Starting small planning for expansion on building modules, one can get to running sooner and allow you to enjoy your time better.
Scale and gauge
There are many modellers who are building railroads in multiple scales or building in different scales which is enough for the most model railroad lovers. The size of the train one can determine the standards which will be set to follow up on the plans.