The present article uses game experiments to understand the dynamics of oligopolistic competition in liner shipping markets. We show how a limited number of carriers, interacting over time, acting independently or grouped into global shipping alliances, are able to effectively and jointly reduce excess capacity. A serious game (called TRALIN) has been designed to this end, mimicking the global liner shipping market where four to five global shipping alliances compete on a set of 12 routes, connecting four ports of call for a few sequential voyages. Carriers are initially subject to low profits due to over-capacity and have to anticipate competitor capacity decisions and vessel deployment simultaneously. Results from 18 experimental games with 4644 decisions were collected and statistically analysed to confirm the main tenets of oligopoly theory and to highlight the existence of a learning effect from successive interactions (rounds in games). Our results suggest that a ‘coordinated’ reduction in capacity is more likely to occur when the number of competitors is limited, but even more when excessive capacity is high, urging the need for cooperation; a learning effect amongst market participants is detected over time. Serious games are flexible tools for improving our understanding of competition, the organization of liner shipping networks, and the role played by global shipping alliances. This tool may help practitioners to understand how over-capacity is evolving within the competitive process, and what factors may influence it. Although voluntarily made simplistic for the purpose of experiments, our design allows one to focus on the main tenets of oligopoly theory as applied to shipping markets.