Double marginalization in the pricing of complements: The case of US freight railroads

Monopolists selling complementary products charge a higher price in a static equilibrium than a single (multiproduct) monopolist would, reducing both the industry profits and consumer surplus. Firms could instead reach a Pareto improvement by lowering prices to the single‐monopolist level. We analyze pricing data of railroad coal shipping in the United States. We compare a coal producer that needs to ship from A to C, with the route passing through B, in two cases: (1) the same railroad owning AB and BC and (2) different railroads owning AB and BC. We do not find that the price in case (2) is higher than the price in case (1), suggesting that the complementary monopolist pricing inefficiency is absent in this market. Our findings are robust to propensity score blocking, causal machine learning algorithms, and difference‐in‐differences analysis. Our results have implications for vertical mergers, tragedy of the anticommons, mergers of firms selling complements, elimination of double marginalization, and royalty stacking and patent thickets.