China is offering high-speed railway (HSR)—the most prominent part of the Belt and Road Initiative (BRI)—to neighbouring member countries of the Association of Southeast Asian Nations (ASEAN). However, those countries’ agencies have been overlooked while discussing China’s eco-political interests in infrastructure cooperation. Thus, this article examines China’s HSR projects in Southeast Asia to identify their geopolitical implications, especially regarding the ASEAN member countries’ impact on China’s regional competition. It focuses on China’s expanding railway diplomacy and the reactions of ASEAN member countries and other major extra-regional countries. It argues that ASEAN member countries are not passive recipients of HSR projects, as they show significant agency toward China. Indeed, HSR projects reveal ‘dynamic interdependence’ between such countries on the one hand, and China as well as other major powers competing for regional geopolitics. China utilizing the BRI to connect ASEAN member countries with its railway network for economic, political, and strategic purposes has resulted in two geopolitical implications: interdependent relations between China and ASEAN member countries, as both sides must collaborate in planning, constructing, and managing the HSR and increased geopolitical autonomy of Southeast Asian countries, as the US and its allies also offer connectivity schemes to contain China. © 2025 Informa UK Limited, trading as Taylor & Francis Group.