From this:

Action News has learned that last week SEPTA police recovered an infrared sensor that was carefully concealed along SEPTA's track bed near 34th and Powelton. The device, a commercially available wireless infrared transmitter made for home security use was discovered spray-painted black and tucked neatly in the trackside ballast. Such devices transmit a signal when something cuts across their infrared beam. And while there is no indication of any specific threat, investigators are concerned because the sensor has the potential to be used as a triggering device...