its tire 24 of flexible material or a guidance failure to eliminate all doubt as regards the nature of the problem detected by the vibrational sensor 5 particularly distinguishing between wearing of or damage to the tire 24 and a guidance failure one uses a proximity sensor 7 in the form of for example an inductive proximity sensor 8 that continually indicates its distance u2014 particularly vertical u2014 in relation to a reference surface this proximity sensor 7 is mounted on the guidance system 2 at a location that enables it to give a relatively interference free indication of the distance in relation to a reference surface u2014 particularly its distance in relation to the upper surface of the guide rail 3 which is hereafter referred to as dimension h and is identified in fig5 in the event of a problem detected by the analysis of the signal from the vibrational sensor 5 and if the value of dimension h is greater than its normal value i e the value corresponding to normal guidance operation of the guidance system 2 then there is a guidance failure namely guide rollers 9 no longer rolling normally on the guide rail 3 through contact of their tires or there is a sensor failure if the value of h measured by the proximity sensor remains nearly constant and close to its normal value and if there is a vibrational alarm one can suspect wearing of or damage to the tires of the rollers in any case each time there is a vibrational alarm at the same time as a.