The flat car roughly follows the Taku Tramway flat, as drawn in an early 1970s Slim Gauge News, and which was in bad shape on the WP&Y. I didn't have the pedestals and 24" axle sets, so the basic car rides on 8-spoke 16" wheels, from an early southern Colorado coal mine car. I had two of those cars and the donor car was a complete wreck. The subframe rides on double springs at each wheel and the springs are NOS springs for an ironworker. The car will handle a load of around 7,000lbs. The subframe has two 6x6 sills which 4x6 end beams. These are treated yellow pine and the sills have double tendons into the end beams, with 3/4" draw bolts holding it together. The upper frame is full dimension 4x6 Douglas Fir sills, which are double tendoned into 6x6 treated YP end beams. The buffer blocks are 4x6 treated YP and they are faced with 3x3x3/8 angles. The outside sills are tied with 3/4" draw bolts, while next to the inner sills are 7/8" tension rods, running through the buffer blocks. The draft beams are full 4x6 DF and they are attached to the buffer block and the inner sills with two 7/8" bolts (old guard rail attachment bolts) and a 1/2" bolt. The center 7/8" bolt runs through 1x3 steel shear blocks. The subframe has four cross beams, which are 4x6 treated YP and every junction of the sills and the cross beams is tied with a 1/2" bolt, so there are 16 of those. The decking is standard 2x8 YP, which I cut lap joints on and then let season. The planking is held on by 40d ring shank pole barn nails, which had the heads ground to resemble cut nails. Those are of course nailed with the long side of the head running with the length of the planks, as this is how cut nails would have been driven to avoid splitting the plank. The subframe is 10'-7" over the end beams and the upper frame is 16' over the end beams and 16'-7" over the buffer blocks. The frame is 6'-9" over the outside of the outer sills and the planking and the end beams are 6'-0" long. Unloaded, the deck is 35.5" above the rail and the center of coupler is 25". The finished car will used replicas of the 1881/1883 D&RG 4-spoke brake wheel and stake pockets. The couplers will be link and pin and the draft gear will be a replica of the draft gear on the Kansas Central box car, in Leavenworth, Kansas, which is a late 1870s car.
That is a lot more than anyone probably wanted to know.
Dave