This article deals with a linear classical approach for the robust output reference trajectory tracking control of nonlinear SISO Lagrangian systems with a controllable(flat)tangent linearization around an operating equilibrium point.An endogenous injections and exogenous feedback(EIEF)approach is proposed,which is naturally equivalent to the generalized propor-tional integral control method and to a robust classical compensation network.It is shown that the EIEF controller is also equivalent,within a frequency domain setting demanding respect for the separation principle,to the reduced order observer based active disturbance rejection control approach.The proposed linear control approach is robust with respect to total dis-turbances and,thus,it is effective for the linear control of the nonlinear Lagrangian system.An illustrative nonlinear rotary crane Lagrangian system example,which is non-feedback linearizable,is presented along with digital computer simulations.