Having been down to the platform on the tour, the photographs in the article are all correct, except the caption for the fourth one down showing the stairs going down from the main station platform, these do not go down to the disused low level platform, they go down to a corridor that links platforms 3-8, and used to be accessible from the stairs going down to Argyle Street. This passageway was (is?) used to provide a controlled queuing system for football supporters going to Hampden Park on Internationals and Cup Finals, kept the supporters and the ordinary passengers apart.
My photographs from my visit are here.
https://www.flickr.com/photos/gateside_and_northbridge/albums/72157661535478189