Why the Union Square subway stop is the only station that relies on automatic steel platform extenders

If you’ve ever found yourself waiting for a downtown 4, 5, or 6 train at Union Square, then you’ve seen them. At the platform edge every 10 feet or so are sets of steel grills. When there’s no train, they don’t move. But when a train pulls in and comes to a stop, the steel grills automatically extend to fill in the gap—which can be up to 13 inches wide, per a 2011 New York Daily News article—between the train doors and the platform. These steel jaws bridge the gap so passengers can onboard smoothly. After the train leaves, the extenders retract until the next train makes its arrival. This hydraulic extension-retraction mechanism must happen hundreds of times a day. The process rarely fails—even though the way they are attached to steel plates on the platform has always seemed a little jerry-rigged. So what’s the purpose of these workhorse platform extenders, also known as platform fillers? It has to do with the way the Union Square subway station was built, and then the demand for…