Rail is the most environmentally friendly and fuel efficient mode of freight transportation. Classification yards are railroads' single-most complex operation. Railroad operations are directly influenced by their performance. In this paper, we present a rail yard simulation framework, or YardSim, and describe how it has been used to model a major hump yard on a Class I railroad in the US. The framework, that models complicated rail yard operations from inbound arrivals, inspection, switching, assembling, brake test, to outbound departures, can be used to pinpoint bottlenecks, identify improvement opportunities in operating practice and yard infrastructure, and assess the impact of changes in traffic volume and service plan. The YardSim provides the flexibility and adaptability to conduct and manage ¿what-if¿ analysis on train plan, yard infrastructure, operating policy, operational strategy, and yard resources such as crews and locomotives. Therefore, the development cycle for a new yard can be reduced significantly.