The photo below shows all the parts in the kit. I highly recommend finding prototype photos on the Internet and in books and/or magazines, because the photos that come with the kit only get you so far.
I started construction with the underframe. It didn't have much flash and was easy to clean up. There are three parts that need to be attached to the underframe. They are shown in the next photo. There is a diagram included on one of the pages of the instructions, but it is very small and hard to decipher. Instead I decided to go on the Internet and see if I could find a better picture of the underframe of an N5c cabin car. The Kohs O-scale N5c Cabin Car Details document has a close-up of their underframe details, which I decided to follow as best I could.
The first step in preparing the underframe was to work on the air brake line. I decided to use 0.019" brass wire (not included with the kit) to represent the brake line, so I needed to drill holes in the various underframe details to route the wire. The drilling has to be done at an angle due to the size of the drills. I used the thinnest one I have. Also, I found it much easier to clamp the underframe to the table using a thin file for added friction and to reduce any clamp damage to the underframe.
I ordered some B.T.S. air hoses (part # 02302), cut them to size and installed the valves in the appropriate locations on the underframe. These parts are not provided with the kit. Note: upon review, at this point in the construction, I was not aware of the end walls that need to be attached to the end of the underframe, so it is waste of time to install air hoses on the ends of the car, because I had to remove them later for final construction. The next photo shows me fitting some 0.019" brass wire to fit between the bolster and the air hose valve. At first I was planning on running one long wire through all the holes I had drilled, but most of the holes were at such an unusable angle, and the constant twists and bends on the wire made it look bad. I decided to just fit one between each underframe detail and glue them into small holes. The wire was attached with superglue.
The three parts shown above are mentioned by name only in the kit's instructions. Having never worked on an underframe of a car, I had no clue which part was which. However, using the above-mentioned Kohs model photo, I was able to match up the parts in the kit. The first part I installed, as shown in the photo below, was placed according to their photo. I glued a short piece of 0.015" brass wire (not included in the kit), to represent a line that runs from the air brake line to the (unnamed) part.
Next, I glued (all using superglue) the other two parts that come in the kit, again determining their placement via the above-mentioned model photo.
The next photo shows several steps. I connected a 0.019" brass wire between the last two parts I installed, approximating its shape and location from the reference model photo. That photo showed an additional part (shown near the bottom of the next photo). I fashioned the part of a piece of leftover sprue from an N-scale kit from years ago (never throw anything away in model railroading!). I filed several levels in the part. I then used some 0.020" styrene, shaped like a long triangle to serve as the foundation for this part. The reference model photo shows the part being attached to two cross beams, so I made those out of some 0.010" styrene. I flattened the area of the new part so that it had more glue surface and built-up the whole assembly.
The final parts were the levers that are attached to the bottom frame members of the car. The kit comes with two of them. However, the reference model photo shows four, and none of them matched the two parts that come with the kit. I decided to add the two parts of the kit to my spare parts box, and made the levers to match the model photo using 0.010" styrene. They each have rounded ends, which I carefully formed using a jeweler's file. Their approximate position and angle of rotation were, again, based on the reference model photo.
The model photo shows a chain between two of the levers. I remembered I had bought some chain years ago from Clover House (part #384), so I cut a piece and superglued it to the two levers. I'm not sure it is visible with the car right side up, but it was fun adding it. This completes all the details I am going to add to the underframe. My objective was not to model the prototype exactly (I have no prototype information for that anyway), but just to give the impression that there is a bunch of stuff under the frame.
I want to go ahead and paint the underframe first before moving on to the next step. I think it will be easier to paint it now rather than wait until it is installed. The photo below shows the underframe airbrushed with Floquil's "Grimy Black". It has just the off-black color I was looking for. This was the first airbrushing I had ever done, and I am happy with the result (it is more black in real life than it is in the photo).