Defining for rms-velosity of rolling stock

The article deals with such important concept as the resonance to three cases: when rolling stock is described by a system of linear differential equations with constant coefficients, when the mathematical model of rolling stock represented a nonlinear system of differential equations, and when in the latter we take into account the multiplicative disturbance from railway, namely its non-equal-elasticity between sleepers. In this case, speak properly and correctly is not about a particular value of RMS-velocity, but on parametric instability areas.