The trade‐off between market power and efficiency gains is central to antitrust analyses of mergers, but empirical evidence quantifying efficiencies remains limited. Using transaction‐level data from U.S. freight railroads (1985–2005), this article quantifies merger‐induced cost efficiencies, driven mainly by eliminating inter‐railroad interchange costs and reoptimization of routing and resource allocation within integrated networks. I develop a spatial equilibrium model with oligopolistic competition to assess equilibrium merger effects. Counterfactual analyses show mergers reduce shipment costs by 12.9% and prices by 8.8%. Markups increase by 7.2%, driven primarily by non‐merging firms reallocating resources away from regions where merged firms achieve large cost reductions.