The U.S. freight rail system, a vital component of the nation's transportation network, faces challenges related to emissions of criteria pollutants and greenhouse gases, impacting both air quality and public health. While the sector currently contributes a small share to total U.S. transportation-related greenhouse gas emissions, there is a growing need to transition to zero-emission technologies to mitigate environmental and health impacts. This shift aligns with global trends where many countries are committing to zero-emission railways, predominantly relying on overhead catenary systems. However, the U.S. lags in this adoption due to financial obstacles associated with private ownership of freight railroads. To address these challenges, the U.S. has initiated efforts at both national and state levels to decarbonize the rail sector. These efforts include integrating rail into the national blueprint for transportation decarbonization and implementing regulations like California's In-Use Locomotive Regulation. Additionally, the technical brief highlights various low- and zero-emission propulsion technology pathways for locomotives, emphasizing the need for a tailored approach based on the specific challenges and opportunities of each technology. Key findings underscore the importance of considering the diverse locomotive segments, such as freight, passenger-rail, and switchers, each requiring a unique technological solution. Interim strategies like partial catenary systems, combining catenary systems with other power sources, are suggested for the U.S. freight rail sector. Moreover, prioritizing segments like switchers and passenger rail for early decarbonization due to their shorter operating distances and lower energy consumption can lead to significant local air quality improvements. Despite the progress, there are significant gaps in data regarding the cost-benefit analysis of zero-emission technologies for rail. However, estimates suggest potential fuel cost savings and substantial health and climate benefits from reduced emissions. As technologies advance and mature in the market, the total cost of ownership for zero-emission technologies is expected to become more competitive compared to traditional diesel powertrains, paving the way for a cleaner and sustainable future for U.S. railways.