I am going to scratchbuild one of these models. To help others (and my "future self") to build a model like this, I have decided to capture as much of the entire process here are possible. This first page holds all the preparation steps I did to get started.
The first step in determining what to build is to narrow-down the model type and the era you are modeling. This will help you figure out which actual prototype model you want to build. The Pennsylvania Railroad had many different kinds of cabin cars. Since I model the summer of 1924, I am looking for any car that was built before that date. Various sources indicate that the N6b was built from 1914 through 1923. So, any version of this car would be suitable.
Studying references, I learned that there were two versions of this car. One had the cupola in the center of the car, and the other had it offset to one end. The built-from-new cars had the centered one, and the rebuild of older cars had the offset one (they simply added a new section to one end of the car). For this build, I am going to build an offset one.
The next phase of the data gathering process is to find any and all scale drawings, prototype photos, and any other text data that might help in determining the basics, such as dimensions, configurations, and maybe even models that were specific to a particular geographic region only. Your ultimate goal is to find a prototype photo (if possible) of a car that you want to model as close as possible. This will then also give you an idea of the road number, and the other lettering of the car that you want to apply to your model. Sometimes the railroad may have made changes to a particular car, so if you compare several photos of the same kind of car, you may notice several differences. This may get confusing, and you may wind up with a model that is a hybrid of several different versions. So, I prefer to do as much research as possible, but then find a good quality photo of one particular car, and then just set out to build a model from that one photo.
For this car, the exterior of the main body is made out of wood siding. I wanted to know what the width of each of these boards was. The center of the two windows on the side are directly over the trucks' bolsters. The trucks' centers are 15 feet apart according to prototype drawings. 15 feet equals 180 inches. From higher-resolution photos I counted 54 individual boards, by taking a copy of the photo and placing a black dot on each board (it is easy to loose count when they are all the same). So, that makes each board 3-1/3" wide.
The next step is to actually make some sort of drawing. This doesn't have to be fancy, but it does provide a nice guide during construction. The side walls are probably the simplest section out of this car to build, so I am going to start there. From prototype drawings I was able to determine all measurements, except for the windows' height, and how far up the wall the windows sat. So, I e-mailed another PRR S-scale modeler who has a model of the SouthWind brass car, and asked if he could measure those two missing distances. He did, and I have incorporated the measurements into the diagram below. To keep things simple, I am using all-inches in the diagrams (this makes it easier to convert to centimeters for those who want to build one in countries where the metric system is used). This diagram shows just the wooden body, not the roof, end platforms, or underframe.
Here is a photo of my informal drawing of the side wall, with all the measurements in inches.
I generally make out of wood those parts that were made out of wood in the prototype, and out of styrene those parts that were made out of steel in the prototype. Since this car's body was entirely made out of wood, I assumed I was going to build it out of wood also. However, another S-scale modeler recommended that I'd use sheet styrene, for ease of construction. After spending some time thinking about it, I realized that there were two problems with going the sheet styrene route. First, Evergreen Scale Models doesn't make a version of their grooved styrene sheet that matches the prototype wood slats I calculated above. Second, I could use a sheet of plain styrene, and make my own grooves using a dental pick of some sorts, but I just wasn't sure that I was going to be able to get a good, consistent look with it (and that is not even considering the pick slipping when I have already done a number of grooves!). So, since I am only going to build one car, I am strongly leaning toward using strip styrene or strip wood to build the exterior of the car.