I couldn't figure out for the life of me how the side frames were to be attached to the trucks. I read the kit's instructions several times. I finally gave up and sent an e-mail to Jeff Smith. He immediately responded and phrased the instructions slightly differently, which eventually gave me the clue. The kit's instructions keep referring to "stretchers" which I didn't know what they were. It turns out that they are the pieces that go in between the sideframes. In the photo below, the "stretchers" are the smaller, T-shaped parts. They go in between the sideframes. The holes in the stretchers match up with holes in the North Yard chassis' trucks, and the kit comes with the appropriate self-tapping screws.
I first had to straighten out the sideframes. They were the most warped parts in the entire kit. The stretchers are mounted to the sideframes by sliding the C-channel-like ends onto the stub protruding perpendicular to the sideframes. The orange line in the photo below indicates how they are to go together. This was not described very well in the instructions, which is why I am providing this close-up detail.
Using 5-minute epoxy, I glued each stretcher to a sideframe, in the same orientation. Note that some of them weren't perfectly straight or perpendicular. This is not an issue, because they will be corrected later.
After the glue had plenty of time to set, I glued two of these sub-assemblies together to make each of the trucks. It is at this point that you can carefully bend and straighten out the parts to make them line up and become a nice rectangle.
After the glue had dried, I slipped them over the wheels of the chassis' trucks (you may have to angle them a bit to get them to go over the wheels, but you don't need to remove the trucks from the chassis), and attached them with the provided screws. There is one screw in the front of the truck and another in the back. I couldn't resist, so I took my newly "tricked-out" chassis to the layout and ran it for a bit. It worked great! Note, however, that the sideframes are all metal and they connect to one another, so an electrical short is possible if the parts touch the wheels. I guess I was lucky in this test run.
I removed them and put a layer of black liquid electrical tape on the side of the side frame near where the wheels could potentially make contact with the sideframes.
The next step is to installed the chassis to the body. It is important to have the trucks' sideframes connected to the trucks, because of clearance issues; it helps to center the chassis in the correct location. This photo shows the chassis positioned in the body, and the chassis mounting brackets shown above and below the model. Note that the kit comes with 6 mounting brackets, four long ones and two short ones. The two short ones are used for installing the chassis to the body (the four long ones are used to connect the body to a chassis other than the one sold by Railmaster Hobbies).
However, before mounting the brackets, the rear screws must be installed first. Make sure to position the chassis exactly where you want it. This is where the side frames being attached to the trucks is important, because you can make sure the trucks clear the pilot step assembly when they swivel. The close-up photo below shows where the pre-drilled hole in the chassis lines up over the mounting tab of the walkway. This provides additional material into which to drill the screw. My kit did not come with screws for mounting the chassis to the body. It would have been a big time-saver if the chassis kit would come with those mounting screws. I had to dig through my collection and eventually found two self-tapping screws that fit the pre-drilled hole.
I then installed the screws. When I then started looking at installing the mounting brackets on the front end of the chassis, I noticed that they didn't fit. They were too far away from the underside of the walkway. When I examined to see what was causing this, I noticed that the screws used to mount the truck towers to the sheet metal of the chassis were sticking out and hitting the underside of the walkway. I looked at the engine from a side-profile and noticed that the chassis' sheet metal was clearly visible. According to the instructions, the chassis plate is supposed to fit up against the underside of the walkway.
To solve this problem, I simply reversed the truck tower mount. The screw mounts into the metal bracket that connects to the tower, so it has to be reversed. That way the screw sticks out under the frame where it won't interfere with anything. This makes the trucks sit slightly lower than before, but the drive mechanism of the chassis has universal joints, so it can handle the positional change.
With this change made, the front mounting brackets fit perfectly, and I was able to glue them into place. The instructions tell you to remove the chassis to mount the brackets, but I wasn't about to risk not quite getting the brackets' location right and not being able to fit the chassis in later. I left the chassis in place, then simply inserted the brackets, with 5-minute epoxy applied to their underside, under the chassis.
The North Yard chassis is a simple sheet of metal with two trucks mounted to it. However, it becomes the foundation for attaching underbody details. These are the battery box, the fuel tank, and the girder. This photo shows the three parts that make up each sub-assembly on the right, and the parts in their approximate position on the left. Note the orientation of the fuel tank (the oval-shaped part). It must be toward the cab (short end) of the engine. The parts, once installed, must be directly opposite of each other, so when you assemble them to the girder, they must be assembled in mirror image, as shown in the photo. Two notes here: first, make sure the back side of the girder is truly flat and straight before gluing the two parts to it (I forgot to do that and had a bit of a challenge getting it straight afterwards); second, be sure to check that the perpendicular mounting bar on the back of the girder is at a 90-degree angle to the back of the girder, because that will be glued to the underside of the chassis later and the girder needs to sit perpendicular to the chassis.
I studied prototype photos to determine where the fuel tank is supposed to sit to determine where to install the fuel-tank-battery-box-girder sub-assembly. The girder's 90-degree mounting bracket comes with a dimple in it and presumably that lines up with the pre-drilled hole in the chassis, but I decided to make sure to position the sub-assembly correctly using 5-minute epoxy only first (I will come back and reinforce this with a screw later on). This photo shows the two metal weights I used to just fit in between the two sub-assemblies to hold them down while the glue set. The front edge of the fuel tank is supposed to be flush with the edge of the walkway above it. My prototype photos show the battery box to be flush (on the outside edge) with the walkway, but I'll let that slide.