In 1972 EMD modified the Blomberg truck for the Dash 2 line by, among other things, replacing the leaf springs with rubber pads. Most if not all Amtrak F40s were delivered with this truck. By the mid-1980s GP units were again leaving LaGrange on trucks with leaf springs and this would continue until the end of GP production. Did the rubber pads not live up to the expectations of their designers? How did the railroads' mechanical departments view the pads instead of the leafs?
I'll bet the rubber was cheaper. It might have had more rigidity than leaf springs. I think the leaf springs ride better. We have both GP38-2 and 40-2s with rubber. the 40s ridge rough, likely because they are heavier.
Maybe they eventually decided that the rigidity of the rubber had a negligible benefit. I'm sure that the harsher ride fell on deaf ears. The later side frames had a additional casting for the shock absorber which connects to the side will. Apparently this is to prevent truck hunting, or at least minimize it. Maybe Preston Cook will chime in as to why it suddenly was needed on the 60 series.