There is an outdoor childrens park in Rapid City, South Dakota that has a little blue train engine with yellow trim. It is only about 4 feet high and six feet long. Odd thing is, there is no motor on it, only an air tank. As I recall, it is an air powered locomotive for use in underground mines. The plaque said it could run for 4 hours on one charge. And it was very safe because there was no combustion engine to cause fires. Mines always have a source of compressed air. It was pretty cool. I have a picture of it around somewhere, just can't locate it now.
Of course, its pretty obvious that the energy required to pump that sucker up would be huge. I don't think it would save any energy at all. It would, however, save on emissions in the area it operated, but not in the area where the compressor got its power from, unless that power was electricity generated by solar, wind, water or nuker.