Here's what Walthers has to say about these -
As covered hoppers evolved in the 1960s, larger cylindrical and rounded car bodies were introduced. This shape allowed more material to be carried and used gravity to force the load toward the bottom discharge outlets. Some of these designs also used pneumatic unloading systems. The continued demand for larger cars led Trinity Industries to introduce their new Power-Flo pressure differential (PD) hoppers in 1987. The design utilized proven technology by combining a pressurized system with rounded side walls. This allowed for increased payload, and reduced the time needed for loading and unloading.