The song sung by Hal's group is an actual song called "Charlie on the M.T.A" It was written in 1949 about the Boston Transit Sytem concerning paying an 'exit fare' when getting off at particular platforms. This is why Charlie "never returned" because he didn't have the nickel needed to leave his destination platform. So he must "ride forever 'neath the streets of Boston."
Here are part of the lyrics:
Let me tell you the story Of a man named Charlie On a tragic and fateful day He put ten cents in his pocket, Kissed his wife and family Went to ride on the MTA Charlie handed in his dime At the Kendall Square Station And he changed for Jamaica Plain When he got there the conductor told him, "One more nickel." Charlie could not get off that train. Did he ever return, No he never returned And his fate is still unlearn'd He may ride forever 'neath the streets of Boston He's the man who never returned.