From August 20, 2012, to September 30, 2015, the Federal Railroad Administration (FRA) sponsored this research for the optimization and testing of a non-invasive locomotive retrofit technology, Locomotive Waste Heat Recovery System (WHRS) dedicated to capturing and converting locomotive waste thermal energy. Optimization and testing activities were further supported via locomotive-simulator and vibration testing to validate the components’ reliability, endurance, and performance. Test results indicated that retrofitting a 4,400 HP locomotive exhaust stack with High-Pressure Heat Exchangers (HiPHEXs) equipped the WHRS with the ability to capture of approximately 420 kW–1,500 kW of otherwise wasted thermal energy. Tests performed to simulate the WHRS components failure indicated that under credible design-basis worst case scenarios, HiPHEXs failure does not negatively impact locomotive engine performance and operations. Converting the captured waste thermal energy into electrical energy results in fuel savings and pollutant emissions reductions proportional to the WHRS efficiency, locomotive duty cycle and fuel price. Overall, test results validated computer models and the performance of key WHRS components, thus enabling further optimization of high-density HiPHEXs with extended surface areas to increase locomotive waste thermal energy recovery without impairing locomotive engine performance. The results of this project indicated a robust technology development pathway enabling railroad operators to lower operating costs, increase public safety by reducing pollutant emissions while decreasing thermal and greenhouse gases (GHG) emissions.