The issue of moving loads is still very active area of research, especially applications related to rail transport as a preferential mode of transport able to reduce the CO2 footprint. Increasing the speed of today's trains to meet demands for increased capacity and passenger comfort by reducing travel times challenges the stability conditions. The instability of moving inertial objects as the anomalous Doppler effect is a physical phenomenon known for years. Several works of the author of this article show that the instability of one moving mass can only occur in the supercritical velocity range, meaning in this context the critical velocity of a moving force, [1]. However, the instability of two moving masses does not respect this condition, [2,3]. This contribution brings a detailed analysis of the conditions implying the instability of two moving masses in subcritical velocity range. The problem is analysed on layered models for railway tracks with continuous supports. It will be defined when the critical velocity can be determined analytically and when it must be replaced by a pseudo-critical velocity. Further, it will be analysed under which conditions instability occurs in the subcritical velocity range for realistic values of moving masses and distances between them. It will also be analysed how the instability is affected by adding a rigid connection between the masses connected to them by means of a spring/damper element simulating in this way a bogie, wheels, and the primary suspension. The contribution will end with practical recommendations for railway design. REFERENCES [1] Z. Dimitrovová. Semi-analytical solution for a problem of a uniformly moving oscillator on an infinite beam on a two-parameter visco-elastic foundation. Journal of Sound and Vibration, 438: 257-290, 2019. [2] Z. Dimitrovová. Dynamic interaction and instability of two moving proximate masses on a beam on a Pasternak viscoelastic foundation. Applied Mathematical Modelling, 100: 192-217, 2021. [3] Z. Dimitrovová. Two-layer model of the railway track: analysis of the critical velocity and instability of two moving proximate masses, International Journal of Mechanical Sciences, 217: 107042, 2022.