The Smart HVAC Control System is a sophisticated microcontroller-based solution designed to manage and optimize the performance of Roof Mounted AC Package Units (RMPUs) in railway coaches. It provides automated control of heating, cooling, and ventilation systems, ensuring that passengers experience consistent and comfortable environmental conditions throughout the journey. The system continuously monitors key parameters such as temperature, humidity, and air pressure, enabling precise adjustments in real time. With integrated diagnostics, fault logging, and remote connectivity capabilities, it enhances system reliability while simplifying maintenance processes. Built to withstand the dynamic conditions of railway operations, the HVAC controller not only improves passenger comfort but also optimizes energy consumption, reduces operational inefficiencies, and supports the transition toward smarter and more connected railway ecosystems.