He has composed music for every season from the eighth onwards, as well as composing music for Calling All Engines!, Thomas' Trusty Friends/On Site with Thomas, and recomposing the American narration of the seventh season. Until the twelfth season, he worked on the songs with Ed Welch. For Day of the Diesels, his son Peter helped compose the songs with him.
Robert Hartshorne was recently commissioned to write a song called "Stronger As One" in honour of Queen Elizabeth II's Diamond Jubilee, sung by Laura Wright.
Robert and his son Peter also often work on the music for various Andrew Marr documentaries, including the recently released documentary series, "Andrew Marr's History of the World".