Basic equations of dynamic stability of a railway vehicle with side swaying, rolling and yawing motions, which travels on a multispan simply supported guideway, were derived by mathematically idealizing a multispan guideway to a circulated guideway system. It was shown that the perturbation equations of above coupled system consisted of a vehicle and a circulated guideway are Hill's equations and the boundary frequency equations of simple parametric resonance, which are available to actual system, were presented in this paper.