I had one of these sets in the early 80s. The Big Loader Construction Set, marketed by Palitoy, was a square track layout with a single groove and a battery-powered set of rugged-looking wheels which fitted into the groove and which would pick up the yellow bodies of either a tipper lorry, dump truck, or JCB-style shovel which at rest would sit in one of two red docking areas. Basically, when the set was built and the wheel unit was switched on, a set of black ball bearings scooped up by the shovel were tipped into the tipper lorry. The wheels would then dock the shovel and pass through onto the lorry which would proceed round the track and tip the balls into a 'Mousetrap Game'-style apparatus in which the balls would weave around a short, convaluted set of chutes into a holding area. The lorry would then make it's way round to a docking station at the other side of the track, the wheels would continue through and pick up the dump truck on the other side of the docking area and the dump truck would make it's way round to the holding area, collect the ball bearings and dump them back into the area where the shovel unit was waiting, before making it's way back to the tipper lorry. Then the whole process would start again, and continue until you got bored of watching it. Admittedly, it wasn't the most interactive of toys once you'd set the thing up - it was just a case of switching the wheels on, placing them into the groove and watching it working. But it worked very well, and kept me out of trouble for a while.