Once we've defined the space available for the model and clarified the desired functions (operational requirements), we need to define some rules that we commit to respecting throughout the design and construction of the model.

Some of these rules are rigid, meaning they don't allow for exceptions once defined. Others are more flexible and can be broken at the expense of only losing some realism and precision.

Strict rules:

• Height of level 0 from the floor - It seems like an easy choice, but it isn't. This is one of the most important. In fact, if the height is too low, it becomes difficult to access from below, and I can guarantee that a few centimeters make a difference in terms of ease of access. It's also true that excessive height will make some parts of the model difficult to see. Therefore, it's necessary to understand what the maximum height (the highest level) will be before setting the height of level 0. In the case of Massa sul Cesi, I adopted 75 cm as the height of level 0 from the floor.

• Minimum distance between adjacent levels - Another very important geometric parameter. On the one hand, we want the esa to be low to limit the length of the ramps between levels. On the other, we need to be able to reach the rolling stock by hand by walking over other stationary rolling stock and/or at least ensure space for the raised pantograph. This parameter was chosen at 11 cm.

• Track module - Maximum length of the trains to be used. This constraint defines the lower limit for the length of station tracks and in-line block sections. In my case, I chose a module of 120 cm in visible stations, which allows for trains with a maximum of three passenger cars in addition to the locomotive. However, in hidden and in-line stations, I adopted a larger module of at least 170 cm. This allows for trains with four carriages to run. In some cases, they simply won't release the entry switch in visible stations, but the train can run and be safely parked in the hidden station. I had to accept this compromise because the space available in the visible stations did not allow for a larger module.

• Curve Radius - Minimum allowed curve radius. This constraint limits the choice of rolling stock that can run on the layout. In fact, some rolling stock cannot negotiate curves with radii below a certain threshold. In the case of Märklin, the problem is less acute, as most rolling stock is designed to run even at radius 1. This is not true for other manufacturers. For example, most Acme trains have serious problems negotiating tight curves. Although in principle I had set myself a minimum radius of 2 (437.5 mm), unfortunately, the reality of the French window leaving only 0.95 m available for the helical ramp forced me to accept radius 1 on the inner track of the double-track line in that part of the layout. Since all my rolling stock runs smoothly with radius 1, I decided to accept this compromise, and so far I've had no problems even with several Italian models converted to the Märklin system. An indirect advantage of this choice is that I was able to use the eight Märklin curve switches I own, which have radius 1. These have improved operating options in the main station and the three hidden stations. While radius 1 is used in some specific locations, these are never visible. In visible locations, I've always tried to use a radius of at least 2, and preferably higher. For example, on the double-track line, as well as in the secondary station, which is in the foreground and clearly visible, I've used a radius of at least 3. In those visible areas, the track spacing is 64 mm, combined with the use of Märklin's slender switches.

• Slope of ramps - Maximum slope of ascending ramps. This constraint is linked to the track layout; the longer the trains, the lower the slope of the ramps must be to allow the locomotives to pull the cars easily. I set myself a goal of not exceeding 3 per thousand and, if possible, staying within 2.5 per thousand, a limit I respected throughout the layout.

• Control type - Vehicle power supply and control methods. The layout is completely digital and based on the Märklin system via Central Station 3, controlled by the iTrain control software on a computer.

• Number of simultaneous trains - Maximum number of trains on the layout and controlled by the PC in automatic mode. This number depends on the number of staging tracks available in the hidden and visible stations. During the design phase, it is necessary to provide a sufficient number of staging tracks in the hidden stations to allow a varied and realistic supply to the visible lines. The theoretical limit for the Massa Sul Cesi layout is 30 (19 in hidden stations and 11 in visible ones). More realistically, 20 trains can run in parallel without any problems.

"Flexible" Rules:

• Epoch - Historical period of reference for trains and setting. The rolling stock I own ranges from Era III to Era VI. Although the project is primarily based on Era IV, both historic and more recent trains can run. The criterion is simple: if I like it, I run it regardless of other constraints.

• Administration - Railway administration of reference. Here too, I was flexible; the layout is ideally set in the Alps, in a border area where Italian trains (primarily, but not only, FS), Swiss SBBCFF trains, German DB trains, and even Austrian ÖBB trains can run.