60000 was very innovative, carrying unusual technology, including a water-tube firebox. this was intended to improve efficiency but the tubes were prone to burst inside the firebox. it is also a compound, expanding the steam once in the inside cylinder and then again in the two outside cylinders. although compounding increases efficiency, it was an extra complication that the us railroads had mostly rejected by the middle twenties. the weight and length of the engine were too much for all but the heaviest and straightest tracks.