The fifth verse of "The Sound of Silence" runs thusly:

> 
> And the people bowed and prayed  
> 
>  To the neon god they made  
> 
>  And the sign flashed out its warning  
> 
>  In the words that it was forming  
> 
>  And the sign said:  
> 
>  "The words of the prophets are  
> 
>  Written on the subway walls  
> 
>  And tenement halls  
> 
>  And whispered in the sound of silence."
> 
> 
> 

There's already a [question about what the neon god is](https://literature.stackexchange.com/q/1372/58). But why does it say that 'the words of the prophets are written on the subway walls'? What does this signify?