Many industries are seeing an increase in concentration, leading to a discussion on the effectiveness of horizontal merger enforcement. The policy debate shows that one of the key arguments put forward when supporting potential mergers is the possibility of realization of merger efficiency gains, specifically in the transport industry. Yet, there exists little empirical evidence on the actual effects of realized mergers on cost efficiencies. We exploit a large and highly debated merger that took place in the French transport industry to evaluate whether a merger between two major transport groups may give rise to merger efficiency gains. We exploit the industry setting to employ a difference-in-differences methodology evaluating the effect of the merger on operating costs of merging transport groups. Our results show that the merger did not lead to any merger specific efficiency gains for the merging parties. Our study relies on the use of several control groups and is robust to a great number of robustness checks as well as to the introduction of heterogeneous treatment effects, depending on the identity of the merging party, the contract type in place, as well as the closeness of competition of local operators. Overall, our study contributes to a growing number of case studies undertaken by economists that can help determine whether horizontal merger policy is being properly enforced.