Method for determining at least one speed in a rail vehicle equipped with at least one first chassis unit, in which - at least one chassis speed measurement variable (V dreh) based on the first chassis unit is provided by means of at least one sensor unit assigned to the first chassis unit, characterized in that - a reference speed characteristic variable (V ref) is formed on the basis of the chassis speed measurement variable (V dreh), - inertial measurement variables (ω b ib, f b) are recorded by an inertial measuring unit, - according to a first operating mode, an inertial speed characteristic variable (V DG) is determined by means of an inertial calculation at least on the basis of the inertial measurement variables (ωb ib, f b) and the reference speed characteristic variable (V ref) in order to estimate a deviation (b gyro, b acc, δv) in the inertial calculation, and is corrected with the aid of this deviation, wherein the inertial calculation is carried out by at least one computing unit (24; 24', 24"), - according to a second operating mode, the inertial speed characteristic variable (V DG) is determined, by means of the inertial calculation, at least on the basis of the inertial measurement variables (ω b ib, f b) and without taking into account the reference speed characteristic variable (V ref), - an abnormal process of providing the chassis speed measurement variable (V dreh) is detected by a detection unit on the basis of a comparison of the inertial speed characteristic variable (V DG) determined in the first operating mode and the chassis speed measurement variable (V dreh), and - the inertial calculation is carried out according to the second operating mode if an abnormal provision process is present. - at least one chassis speed measurement variable (V dreh) based on the first chassis unit is provided by means of at least one sensor unit assigned to the first chassis unit, characterized in that - a reference speed characteristic variable (V ref) is formed on the basis of the chassis speed measurement variable (V dreh), - inertial measurement variables (ω b ib, f b) are recorded by an inertial measuring unit, - according to a first operating mode, an inertial speed characteristic variable (V DG) is determined by means of an inertial calculation at least on the basis of the inertial measurement variables (ωb ib, f b) and the reference speed characteristic variable (V ref) in order to estimate a deviation (b gyro, b acc, δv) in the inertial calculation, and is corrected with the aid of this deviation, wherein the inertial calculation is carried out by at least one computing unit (24; 24', 24"), - according to a second operating mode, the inertial speed characteristic variable (V DG) is determined, by means of the inertial calculation, at least on the basis of the inertial measurement variables (ω b ib, f b) and without taking into account the reference speed characteristic variable (V ref), - an abnormal process of providing the chassis speed measurement variable (V dreh) is detected by a detection unit on the basis of a comparison of the inertial speed characteristic variable (V DG) determined in the first operating mode and the chassis speed measurement variable (V dreh), and - the inertial calculation is carried out according to the second operating mode if an abnormal provision process is present. - a reference speed characteristic variable (V ref) is formed on the basis of the chassis speed measurement variable (V dreh), - inertial measurement variables (ω b ib, f b) are recorded by an inertial measuring unit, - according to a first operating mode, an inertial speed characteristic variable (V DG) is determined by means of an inertial calculation at least on the basis of the inertial measurement variables (ωb ib, f b) and the reference speed characteristic variable (V ref) in order to estimate a deviation (b gyro, b acc, δv) in the inertial calculation, and is corrected with the aid of this deviation, wherein the inertial calculation is carried out by at least one computing unit (24; 24', 24"), - according to a second operating mode, the inertial speed characteristic variable (V DG) is determined, by means of the inertial calculation, at least on the basis of the inertial measurement variables (ω b ib, f b) and without taking into account the reference speed characteristic variable (V ref), - an abnormal process of providing the chassis speed measurement variable (V dreh) is detected by a detection unit on the basis of a comparison of the inertial speed characteristic variable (V DG) determined in the first operating mode and the chassis speed measurement variable (V dreh), and - the inertial calculation is carried out according to the second operating mode if an abnormal provision process is present.