As you've read in the past, Southern Pacific's W-50-3 ballast gondolas were neat cars. The cars, built in 1911, used a patented design from the Rodgers Ballast Car Co. to make them useful in many different kinds of maintenance-of-way service. This "Hart Convertible" design looked like a normal gondola, and could be used for hauling ties or other loose material. Open some doors in the gondola's bed, and you'd expose a hopper, ready to dump ballast between the tracks. Close the hopper doors and unlock side dump doors, and you could push dirt out of the gondolas onto the sides of the track. Close the side dump doors and remove the end bulkheads, and you could easily pull rail out of the car for track-laying.
By 1926, plans show that the SP started removing the side dump doors, replacing them with solid wood sheathing on each side of the car. This change also cut the capacity of the car, and also removed the locking mechanisms that held the side dump doors shut. The SP didn't remove the brackets on each post that held the side locking mechanisms - the brackets also supported steel rod holding the car posts to the frame that probably kept the sides square. As a result, the modified cars had odd, unused castings on each post of the car.
SP kept using the Hart gondolas into the 1950's. A hundred of the original five hundred cars were still on the roster by 1950. By 1955. all the W-50-3s were gone. I've heard stories of cars abandoned along the Cuesta Grade near San Luis Obispo, and trackside in the Oregon Cascades.
If you'd like one of these modern cars for your layout, check out the Dry Creek Models website for photos and ordering details.