To improve railroad safety and efficiency, the Office of Research and Development of the Federal Railroad Administration (FRA) is running a project to develop and demonstrate an On-Board Monitoring Systems Concept (OBMSC) for freight trains. The project scope includes onboard detection of hot bearings, bearing defects, vehicle, ride quality, wheel tread defects, and derailed wheels. This paper presents an analytical model to detect derailed wheel conditions. In the model, an idealized wheelset with associated sprung and unsprung vehicle masses running on crossties is simulated using LS-Dyna software. Track structure (i.e., ties) ballast/subgrade, and soil are represented as linear elastic systems. This paper identifies wheelset vertical acceleration magnitude and associated frequencies for a derailed wheel for empty and loaded car conditions at various operating speeds. The research shows that the predicted wheelset acceleration magnitude for a derailed wheel overlap with those resulting from wheel tread defects, such as wheel flat, shells, and built-up tread. To differentiate between a derailed wheel and wheels with tread defects, a set of criteria is formulated based on amplitude and frequency ranges. Based on the analytical results from the derailed wheel model and field-tested results of revenue service wheels with tread defects, it is established that the OBMSC bearing adapter acceleration (BAA) can be used to detect a derailed wheel and conditions communicated to the train crew or other appropriate parties.