A continuously advancing track maintenance machine for compacting ballast comprises a self-propelled machine frame supported on the track for continuous advancement, a power-driven and vertically adjustable track stabilization assembly mounted on the machine frame and comprising rail engaging roller tools, a drive for spreading the roller tools into engagement with the gage sides of the rails, and vibrating means for imparting oscillations to the roller tools in a direction extending substantially in a horizontal plane transversely to the longitudinal machine frame extension whereby the roller tools engaging the rails transmit the oscillations to the track, a device for measuring the amplitudes of the horizontal oscillations, and a reference system for monitoring the track level between an actual level of the track and a desired level thereof.