Advances in voltage-source converters (VSCs), as well as their successful application in VSC-HVDC systems, have motivated growing interests and research in medium-voltage direct current (MVDC) traction power systems (TPSs) for high-speed rail (HSR) applications. As an emerging power-converter-based infrastructure, this study reviewed developments that shape two key evolving pieces of equipment—namely, high-power traction substation (TSS) converters, and power electronic transformers (PETs)—for MVDC TPS as well as prospects for smart grid (SG) applications in the future. It can be deduced that cost-effective and robust high-power TSS converters are available from hybrid modular multilevel converters (MMCs) for enhanced performance and fault-tolerance capability. In addition, silicon carbide (SiC) MMC-based PETs with input-series-output-parallel (ISOP) configuration are present for greater weight/size reduction and efficiency for MVDC rolling stock design. Finally, the implementation of a smart MVDC TPS incorporating a sophisticated railway energy management system (REM-S) based on the smart grid principles is feasible in the future, with numerous benefits. However, there are related challenges, like knowledge gaps on these technologies, the high costs involved, and lack of standardization to overcome to realize widespread future commercial deployment.