in this case the continuous advancement of the railcar is compensated by the free or controlled oscillation of the columns 56 about their own articulations 57 once a preset oscillation amplitude corresponding to a preset work time of the roller 52 has been reached the jacks 58 and 59 are activated in sequence and respectively cause the lifting of the roller out of the prepared seat s 2 and the reverse oscillation of the columns 56 which return to their initial position to start a new working cycle in the oncoming seat to be prepared the strike element 61 arranged immediately downstream the rotating element 51 is constituted by a vibrating mass 62 supported by a frame 64 by means of a counterframe 63 the vibrating mass 62 is capable of executing longitudinal horizontal movements parallel to the direction of motion of the railcar with respect to the frame 64 by means of retention and sliding guides not illustrated interposed between said counterframe and said frame in turn the frame 64 is supported and guided by vertical uprights 65 with respect whereto it can perform vertical lifting and lowering movements a jack 66 controls the horizontal movements of the counterframe 63 and.