Adler’s first major project was a new type of flashing signal for grade crossings. At the time, many cars didn’t bother to stop at railroad crossings, with the unsurprising result that about 1,500 people were dying in car-train collisions every year. The eventual solution was to eliminate grade crossings wherever possible by placing rail lines above or below the road. In the meantime, the American Railway Association (ARA), the trade group for the U.S. railroad industry, directed its member companies to install some sort of flashing light at such intersections.
The system that Adler designed was triggered automatically by the train as it approached the intersection. Two lights would flash in an alternating pattern, known as a wigwag, which mimicked the way a man swinging a lantern might warn oncoming cars. Adler’s flashing signal received the ARA’s endorsement, and more than 40 railroad companies adopted it. (Source)