In this paper an automated algorithmic method is presented for the dynamic analysis of geared trains/transmissions. These are treated as a system of inter-connected flexible bodies. The procedure developed explains the switching of constraints with time as a result of the change in the contacting areas at the gear teeth. The elastic behavior of the system is studied through the employment of 3-D isoparametric elements having six degrees of freedom at each node. The contact between the bodies is assumed at the various nodes, which could be either a line or a plane. The kinematical expressions together with the equations of motion using Kane’s method, strain energy concepts are presented in a matrix form suitable for computer implementation. The constraint jacobian matrices are generated automatically based on the contact information between the bodies. The concepts of the relative velocity at the contacting points at the tooth pairs and the subsequent use of the transmission ratios in the analysis is presented.