The internationally accepted method for assessing track stability is the calculation based on Meier's theory. A critical point is the inclusion of the equivalent track bending stiffness. Practical measurements have often given contradictory results in determining this, so the authors present a purely theoretical determination in this article. For this purpose, the Nemesdy theory, which is used in Hungarian practice, is invoked. By applying this theory, the paper introduces an auxiliary factor that allows us to calculate the value of the inertia of the two rails on the vertical axis. Stopping at this point in the theoretical derivation and recognizing the possibility, an iterative solution is proposed by which the magnitude of the substitute inertia can be considered in Meier's calculation without performing the necessary calculations using Nemesdy's theory.