I don't think this question got as much attention as it deserved back in October, so I'm reviving it. I, too, think "Signal Song" is a wonderful concept - but it doesn't have to be a song. It could be any other work of art: a poem, a film, a book, or a painting. So, I'd prefer to call it a "Signal Work."

As I see it, a Signal Work is something that obsesses you at a particular point in your life, because its theme is somehow relevant to an important challenge you are facing. Sometimes you are at a loss to explain how it is relevant at the time. Explaining it might require insight that doesn't come to you until years later, after you have solved the problem of how to manage that aspect of your life.

Some works of art work on an unconscious or semi-conscious level, helping you express something you otherwise wouldn't know how to express - or are afraid to express.

Some works that obsessed me as a youth:

Edmond Rostand's play, "Cyrano de Bergerac."
T. S. Eliot's poem, "The Love Song of J. Alfred Prufrock."
Nicolas Roeg's film, "Walkabout."

To be honest, I don't think any music has moved me half as much as those, but some songs of that period that come to mind are:
Paul Simon's "Sounds of Silence" and "Bridge over Troubled Water."
Stephen Stills' "For What It's Worth."
Graham Nash's "Teach Your Children."