If you’ve ever ridden a subway, you’ve probably experienced a bump in the tracks that could send your coffee onto another rider. George Lederman, doctoral candidate at Carnegie Mellon University, wants to fix that, though you may never notice his work. Piezoelectric sensors fixed to train cars with electrical tape are at the heart of Lederman’s monitoring equipment. They measure changes in pressure, acceleration, temperature, strain and force by converting these changes into an electrical charge.  “The car vibrates and the amplitude of that signal corresponds to the amount of vibration the car senses,” said Port Authority engineer Dave Kramer, who’s worked closely with Lederman. Each car bears one sensor on its underbelly and two more locked in the cabinets near the driver’s booth. They communicate with a GPS antenna atop the vehicle through tiny computers, so when a spike in the data pops up, Lederman can determine the coordinates of bumps along the track.