This paper proposes a distributed cooperative control strategy to balance the voltages of the series-connected ultracapacitors which are the power supply of energy storage light rail vehicles, and their performance is of great significant. Taking each ultracapacitor cell as an agent in the cyber layer, the voltage equalization problem can be formulated as the cooperative tracking problem of multi-agent systems. The sparse communication network among the series-connected ultracapacitors can be modeled by a directed graph. Based on the nearest neighborhood rule, a bounded distributed control law is designed with the aid of a S-like saturation function and the control gains can be tuned according to the system performance. Assuming that the digraph has a directed spanning tree, the asymptotically stability of the overall close-loop system is rigorously proved by the Lyapunov function integrating the Lasalle invariant principle. Simulations verify the effectiveness and feasibility of the proposed cooperative control strategy.