The Harken mains'l traveller track is bent horizontally to conform to the shape of the taffrail. This allows the track to be mounted on the aft trailing edge of the taffrail leaving room for the center single swivel ratchet block with cam cleat to be fix mounted on the taffrail centerline in front of the track. But the taffrail is also concave which makes the ends of the track lower than the center. In fact the track ends should be bent upwards so that the mains'l leech does not tighten when the traveler car is released to run downhill. In the photos below you can see the long radius wedges that have been bolted and glued to the taffrail to give the track the proper vertical shape.