The invention relates to a movable preparation machine, in particular a screening machine, for ballast gravel for railways, comprising a supporting structure extending in the longitudinal direction of the rails, a running gear, a device for receiving the gravel to be processed, longitudinal conveying means for transferring the gravel to a machining station, in particular a sifting station, and discharging means for prepared gravel, and optionally for preparation waste. The object of the invention is to create structures for relatively small and appropriately economical preparation machines for ballast of the track, which machines can be moved with ease and are flexible but are highly efficient and labour-saving, in particular for use when scattering and working on numerous relatively short track sections. The invention is attained with a preparation machine comprising a running gear (FW) with at least one wheel set (SPR) with flanges for rail mounted travel, and at least one wheel set (ADR) adhesion-guided for non-rail travel, as well as a travel change-over device (FUS) for selective activation of said wheel sets.