  We present a combined theoretical approach to study the nonequilibrium
transport properties of nanoscale systems coupled to metallic electrodes and
exhibiting strong electron-phonon interactions. We use the Keldysh Green
function formalism to generalize beyond linear theory in the applied voltage an
equation of motion method and an interpolative self-energy approximation
previously developed in equilibrium. We analyze the specific characteristics of
inelastic transport appearing in the intensity versus voltage curves and in the
conductance, providing qualitative criteria for the sign of the step-like
features in the conductance. Excellent overall agreement between both
approaches is found for a wide range of parameters.
