A mathematical dynamic model of railway conventional truck is presented with 12 degrees of freedom equations of motion. The presented dynamic system consists of conventional truck attached with two single wheelsets in which equipped with lateral, longitudinal and vertical linear stiffness and damping primary and secondary suspensions. This investigated model governs lateral displacement, vertical displacement, roll yaw angles of each of wheelset and the lateral displacement, vertical

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... t, roll and yaw angle of conventional truck. Kalker's linear theory has been adopted to evaluate the creep forces which are introduced on rail wheels due to rail wheel contact. The railway truck mathematical equations of motion are solved using fourth order Rung-Kutta method which requires that differential equations to be transformed into a set of first order differential equations. The transformed state space equations are simulated with computer aided simulation to represent the dynamic behavior and time solution of dynamics of conventional truck moving on tangent tracks. Influences of the geometric parameters of the rail wheel such as wheel conicity and nominal rolling radius on the dynamic stability of the system are investigated. It is concluded that the geometric parameters of the rail wheel have different effects on the hunting instability and on the change of the critical hunting velocity of the system. In addition critical hunting velocity of rail trucks is proportional inversely with the square roots of wheel conicity but high critical hunting velocity obtained by increasing the nominal rolling radius of the rail wheel.