To improve the safety of passenger train exterior side doors, this SBIR project develops a low-cost Wireless Sensing and Warning System (WS2), a mesh network of advanced magnetic door position sensors that provides continuous monitoring and issues real-time warning to the train operator when a door is not properly closed.WS2 aims at retrofitting legacy passenger trains that do not possess a door safety system.Compared with other potential technologies such as computer vision, the proposed sensor system not only offers higher detection accuracy and robustness, but also consumes far less energy, thus enabling a battery-powered continuous monitoring system that makes the retrofitting far more cost-effective.The Phase I objective is to demonstrate the feasibility of accurate detection and warning of a door that is opened by 1/8. Major tasks include (1) collection of information about legacy passenger train door systems and constraints, (2) functional design and prototyping of a proof-of-concept sensor system incorporating advanced sensing algorithms to minimize measurement errors, and (3) feasibility tests of the prototype using mockup sliding and swing doors. The project team has extensive experience with creation of sensor-based technology solutions to challenging problems including those related to passenger railcar door safety.