Large freight railroads in the United States typically own several thousand locomotives to power the thousands of trains they operate every day. When trains run late or are cancelled, locomotives are not available to run other trains. These locomotives also visit shops for fueling, scheduled maintenance and repairs. With the variability of train and locomotive operations, railroads have questions about the optimal locomotive fleet size and mix to achieve a specific on-time train departure performance; ideal shop locations, capacities and capabilities, and the impact of different operating practices on locomotive productivity and utilization are also measured. These questions can be answered through a simulation system that realistically models the movement of locomotives in the train network. CSX Transportation hired Optym to build this simulation system.