He has composed music for every season from the eighth season until the nineteenth season, 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. In Tale of the Brave, he composed and performed Monsters Everywhere.
He was also 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".
He has also composed a song for one of HiT Entertainment's other brands, "Fireman Sam".
After Sodor's Legend of the Lost Treasure, Robert and his son's contract with HiT Entertainment had ended and the executives wanted to use someone new. Chris Renshaw took their place as composer from The Great Race onwards.
- Monsters Everywhere
- Really Useful Engine (with The Kidmore Children's Choir)
- Troublesome Trucks (with The Kidmore Children's Choir)
- Down by the Docks (No Help At All)
- Music Videos - list of Thomas & Friends compositions by Robert Hartshorne.