Complex sentence: In 1947, Bruno Walter re-united with the orchestra to conduct it when it appeared at the first Edinburgh Festival; one work they performed was Mahler's song cycle "Das Lied von der Erde".
Split into simpler sentences: In 1947, Bruno Walter reunited with the orchestra as conductor when it appeared at the first Edinburgh Festival. They performed a single work, Mahler's song cycle "Das Lied von der Erde".