Vehicle suspension systems including a truck and a spring system for an axle are provided. In one example, a truck includes an unpowered axle spring system that is configured to generate an overall unpowered axle separation force between a truck frame and an unpowered axle carrier. The overall unpowered axle separation force has a rate of decrease that increases past a first separation distance between the truck frame and the unpowered axle carrier. A powered axle spring system is configured to generate a powered axle separation force between the truck frame and a powered axle carrier. The powered axle separation force has a rate of decrease that decreases past a second separation distance between the truck frame and the powered axle carrier.