Companies were dispatched to the above address for the bldg fire. Arriving companies had heavy black smoke billowing from the basement of the bldg. Crews led off with an 1-3/4 line to the interior. Command notified fireboard of an active gas leak in the bldg requesting Peco gas with a rush. After shoveling some snow, the gas shut off was found and the gas to the bldg shut off. It was discovered that what was burning was underground transformers in a small room to the front of the bldg. Crews stood by with handlines and dry powder while Peco got power to the transformers secured. Crews were o/s for over 2 hrs.