Atlas produced a very nice looking Hart ballast Car. Unfortunately they issued it with the incorrect trucks and left out much of the interior frame. The lack of frame makes the car look like a pair of high-water pants. I used 3/16th styrene channel to fill in the frame on my car. You have to do a little trimming to match the angles on the underframe and to fit over the bolster. The rectangular supports on the side of the kingpin must be trimmed also - easily done with a Dremmel cutoff wheel. DO NOT trim to two pins on the "A" end of the car that keep the trucks from rocking. DO NOT INSTALL THE CENTER FRAME BETWEEN THE BAYS (LEFT SIDE OF THE TOP PHOTO) AS I DID IN THE PHOTO. Steve Orth directed me to a photo of a car laying on its side and they are not there.
I built the bolster by cutting a length of .020"x.156" styrene. It should be long enough to extend from the inside portion of the channel to the end of the angle in the molded "mini" bolster. Cut the piece of styrene length wise from corner to corner forming to equally sized triangles. Glue two on each side of the frame - about 1/8 inch apart - On the edges of the razed portion of the molded bolster. You'll have to trim the top of the triangle slightly to fit. Finally Use the .020"x.156" styrene to form the top plate on the bolster. I use some pliers to pre-bend the angles. A combination of liquid styrene cement and CA to glue everything up. I also found that a portion of the top plate needed to be trimmed so the truck can sit correctly on the kingpin.
You can see a big difference in the modified (right) and the unmodified (left) cars. Other changes I made included #58 Kadee couplers, new trucks, and Intermountain wheel sets.