In 1972, when the -2 series locos were introduced, most of the North American RR's were changing to high friction composition shoes from cast iron. The change from elliptic springs to the rubber compression pad was made to facilitate the brake rigging with composition shoes, which needed only one shoe per wheel. To operate the shoes on one side of the truck with a single brake cylinder, a slack adjuster that ran underneath the spring plank was connected between the live and dead levers. To keep the slack adjuster high enough above the rail, the swing hangers were shortened and the spring plank raised which did not allow enough height to use the original elliptic spring. The rubber compression spring was substituted to take it's place but it only had limited static deflection (compression under load), much less than the elliptics, which negatively affected the vertical ride quality. Eventually, the elliptic spring technology progressed to where a shorter height elliptic spring could be manufactured to carry the load and restore much of the lost static deflection. This was introduced around 1984, IIRC. So the old elliptic spring, which typically had 9 leaves per half, was replaced by the low profile elliptic spring, which has typically only 3 much thicker leaves.
It's common to see single shoe brakes on the swinghanger truck where a rebuilder or the RR has simply retained the two brake cylinders on each side and added a very short slack adjuster between the live lever and a bracket welded to the safety strap that is there to support the loco if a swinghanger breaks. However, while this allows retention of the original leaf springs and swinghanger, it puts a load on the safety strap it was never designed to carry.