the upper end of the housing 32 which is a box shaped frame having at least one open side 36 enabling access to the interior thereof and which contains a vacuum unit 38 including a receptacle 42 for receiving particulate matter or debris which is aspirated by the vacuum unit from the surface of the overhead transport rail 12 the aspirating action for cleaning the overhead rail surface by the vacuum unit 38 is effected through a suitable hose 40 which is connected to respectively the receptacle 42 of the vacuum assembly and to a suctioning nozzle aperture provided on the nozzle trolley structure 30 and which travels in the interior space of the c shaped overhead transport rail 12 as elucidated further on hereinbelow as illustrated in fig2 the drawings showing a representative section of the oht support rail 12 the nozzle trolley structure 30 comprises a block shaped element 50 having a first pair of guide rollers or wheels 52 rotatably journaled on a suitable support shaft not shown and traveling within the lower confines of the c shaped oht rail in contact with the rail surface 54 the block element 50 further contains additional guide wheels 56 which travel in contact with the lower front surface 58 of the oht support rail 12 and engage the surface in a generally close guiding manner in cooperation with guide wheels 52 the block element 50 is equipped with further upper guide wheels 60 as.