I must have went down the block of w 31st St 8-9 Ave thousands of times over many years, but it was not until I read Conquering Gotham that I realized this building is really the only part left of the old Penn Station.
Yes it is! That's was the old power plant or something to that effect. I believe they have steam pipes in there that provided steam heat for the station and the trains as well.
There is several other artifacts of the old temple still standing. Many are in plain sight many others are hidden behind walls and/or in off limits areas.
As for Above Ground not including her eagles, The power plant is all that's left. I'd like to know if that building was built of the same stone at the station? If so a good cleaning would give the image of what the station might have looked like had it survived to be restored.