There isn't nor was there ever, a "breakaway wall." What has led to this "urban legend" is the simple reality that, as the terminal trackage was constructed, there is in fact only a wall under the terminal --- or under 42nd St., I forget which --- separating the terminal trackage from the IRT line.
Don't forget that the original 1904 IRT line came up under 4th Avenue, turned west under 42nd St [the present day shuttle line], then north under Broadway. When the Fourth Ave. - Lexington Ave. line as we know it today was constructed as the through routing there were some track reconfigurations and eliminations to the original routing [the one that became the shuttle] that did indeed result in some "unused tunnels." If you ride those subway routes you have no doubt walked though one! If, as you leave the shuttle at the GCT end, you do not go up the stairs to the terminal but instead take the walkway that leads you directly to the Lexington Ave. line, you are walking right through one of them. AND --- and you walk that walk, the wall on your left is the wall that separates you from the CGT Loop tracks.
Writings about the railroad and GCT have been known to point out that it would be theoretically possible for a train coming into GCT to continue downtown using the IRT tracks, if that wall were "broken down." Of course, "theoretically" is a very long way from doing it!!!! I imagine these statements have gotten inflated into the tales of "breakaway walls, and so on.