Here’s what John Henken wrote about the piece:
A genuine prodigy of wide-ranging talent and interests, Saint-Saëns composed in almost every form and medium then imaginable. One of his musical indulgences was La Trompette, a Parisian amateur chamber music society that Saint-Saëns did much to professionalize. Its name was a sort of in-joke and its founder, the mathematician Émile Lemoine, had begged the composer for years to write something that would include trumpet with the society’s usual strings and piano.
Saint-Saëns had always replied with a joke about the unusual combination of instruments, but at the end of 1879 he offered Lemoine a Préambule for trumpet, string quartet, bass, and piano as a New Year (or belated Christmas) gift. Lemoine promptly had it performed and Saint-Saëns was inspired to complete the work – originally titled Suite – with three more movements. (The somber Intermède was at first labeled Marche funèbre.)
Two things probably prompted Saint-Saëns’ reconsidered interest. One was the continued development of the valve trumpet and the concurrent development of virtuosos on the instrument. The Besson company in Paris (and London) was a leader in the field, and created higher pitched instruments, such as the E-flat trumpet. Xavier-Napoléon Teste, the principal trumpet of the Paris Opera, played Besson instruments, and he joined Saint-Saëns (at the piano) for the premiere of the complete Septet in December 1880, accompanied by a double quartet and bass. (Though the trumpet part, filled as it is with simple scales and triadic fanfares, hardly suggests virtuoso valve work.)
The other was a renewed Parisian interest in older music, particularly of the Baroque era. There are little flourishes of counterpoint in the Septet, particularly the second half of the Préambule, but this manifests itself here mostly in the clarity and simplicity of the style, textures, and harmonies – a work Neoclassical before its time.
The Septet was extraordinarily popular and influential. The composer quickly arranged it for piano trio and his protégé Gabriel Fauré arranged it for piano four-hands, and numerous other versions soon followed. The piano part is quite brilliant at times, and Saint-Saëns played it himself many times, often with the reinforced strings as in its premiere.