From the September 1954 issue of Applied Hydraulics (the original title of H&P).
Three major design problems are solved in the construction of a hydraulic power system used to dump slag-loaded railroad cars. The design keeps two cylinders moving together regardless of varying loads, eliminates the possibility of shock in the system because of the large mass being handled, and provides rigid piping on oscillatingcylinders.
Beyond those considerations, several other requirements had to be met, including protection from the outdoors and simple "up-down" pushbutton control had to be properly interlocked for safe and efficient operation.
Synchronizing the motion of two cylinders is always a difficult problem. Rate of travel depends on the volume of oil delivered to a cylinder, and moving them together requires equal oil flow to each cylinder. Leakage, pump slip, changing work loads, and varying friction loads affect oil delivery. On the car dumping system, uneven loading is almost inevitable, yet the jack cylinder must move together to prevent the car body from twisting.
To ensure synchronization of the jack cylinders, each one is powered from a separate variable and reversible volume pump. In addition to offering high volumetric efficiency, the pumps allow the necessary incremental volume adjustment needed for synchronization.