would normally carry the pick up means 12 electrical connection is established between the cable 34 of the pick up means 12 and the electric rails 13 by way of sliding contacts 14 the slot 11 which is provided in the wall 17 of the casing of the duct section is sealed with the aid of separate seals 15 16 the seals 15 16 take the form of thin resilient wear resistant steel strips having a width somewhat greater than the slot 11 the seal 15 is located inside the casing and engages on the inside surface of the wall 17 to cover the width and length of the slot 11 interiorly conversely the seal 16 engages on the outside surface of the wall 17 to cover the width and length of the slot 11 exteriorly both seals 15 16 are held in sealing engagement with the wall 17 by means of permanent magnets 18 located on the edges of the slot 11 the pick up means 12 is adapted to deflect and lift the seals 15 16 away from the slot 11 locally as the pick up means progresses along the duct section in question to permit access to the rail 11 the pick up means 12 employs inter alia a shaped member or slide piece 20 which engages in the slots 11 to run along the duct sections the slide piece 20 is depicted separately in fig5 and as shown it has.