It's the air brake on the wagon. It's a brand new technology that stops the wagon. Instead of shouting "Woaahh" and pulling the reins, pull the handle. The wagon wheels stop, the horses are choked (not enough to be asphyxiated). The horses do not expect this and halt. It was going to be patented, but PETA got in the way with their vegan scantily clad girls. The patent officers were wooed, and thus it was stopped from being produced en masse. This technology somehow lives on in Trainz, much to activists' dismay.