American Music Trivia

Who wrote this quote and to what famous work was this composer referring?

"It was on the train, with its steely rhythms, its rattle-ty bang, that is so often so stimulating to a composer – I frequently hear music in the very heart of the noise.... And there I suddenly heard, and even saw on paper – the complete construction of the Rhapsody, from beginning to end. No new themes came to me, but I worked on the thematic material already in my mind and tried to conceive the composition as a whole. I heard it as a sort of musical kaleidoscope of America, of our vast melting pot, of our unduplicated national pep, of our metropolitan madness. By the time I reached Boston I had a definite plot of the piece, as distinguished from its actual substance."