From reading your post, I believe that you have already identified why these awful memories have returned in a dream (nightmare?). One of the things that we know about music is that it is SO powerful. When one hears a song repeatedly (as you did when Kate Smith sang "God Bless America" night after night), it leaves an imprint in every cell of your body. Many people who can't remember critical information about their own lives that happened just yesterday (and trust me, I'm one of them!) can recite the lyrics to songs that they'd heard when they were kids. Moreover, most of us associate specific songs with significant events in our lives (our first love, a grandparent's death, etc.).
My hunch - from your post - is that "God Bless America" (however consciously or unconsciously) takes your right back to 1951...to a time when you were very sick and very vulnerable. (And being raped in a hospital when you were a little kid is no small event, I might add!) I'm no psychologist, but my sense is that your accident with the subway train (yet another traumatic event) has less to do with these flashback than the song. Music is SO very powerful....