A 2.0 -kg toy locomotive is pulling a 1.0 -kg caboose. The frictional force of the track on the caboose is $0.50 \mathrm{N}$ backward along the track. If the train's acceleration forward is $3.0 \mathrm{m} / \mathrm{s}^{2},$ what is the magnitude of the force exerted by the locomotive on the caboose?