The operating shaft for the magnets goes out both sides of the baseboard and is finished with a small knob. A half turn gives full travel, and it can be operated from either side of the layout.
The cam is placed so it can pass top centre (be turned all the way round) without stressing anything - nearly idiot proof. The bearing near the cam is needed because over the 2 foot width of the boards these 4mm dia. steel shafts are not rigid.