I have a couple thoughts on this. I believe your observation about the constant weight of a caboose is a significant factor affecting the truck suspension design. Spring rate is also a factor. The spring rate is the rate the spring load increases per unit of distance that the spring is compressed.
With a coil compression spring of a given diameter and wire size, the rate varies with the number of coils. The greater the number of coils, the less steep the spring rate. In other words, a given applied load will compress the spring with more coils a greater distance than the spring with less coils. Or in still other words, a shorter coil spring is stiffer and harder riding than a longer spring. The shorter spring absorbs shocks in a shorter travel distance than a longer spring.
There is only so much room for the height of coil springs, so if you want a softer riding spring, an option is to revert to a different type of spring that has a less steep rate. I am guessing that the leaf spring set would have a flatter rate for its height than a coil spring. If so, it would fundamentally be softer riding than coil springs.
Another factor in suspension is dampening, which is friction to impede the spring movement. When a spring absorbs a shock, its job is done. But it tends to keep reacting by bouncing until the reaction to the shock is dissipated. The friction of dampening impedes the bouncing that follows a shock. On highway vehicles, the dampening is performed by the shock absorbers. On freight car trucks, the dampening is performed by a unique mechanism that creates friction between the truck bolster and side frames.
Leaf springs have inherent self-dampening by the friction created as the leafs slide against each other as the spring is compressed. Closed coil torsion springs also have self-dampening. However, coil compression springs have no self-dampening, so if dampening is desired, it must be added as a separate device or system. So the self-dampening effect with leaf springs is an advantage.
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