Until 2007 with the publication of Thomas the Tank Engine: The New Collection, very little information about the working relationship between Christopher Awdry and Clive was known. It was then, however, that Christopher commended Spong's "superb" work. And because of this, Clive remained the sole illustrator of Christopher's books and provided artwork for more Railway Series books than any of his predecessors. This was likely out of enthusiasm for the characters which he had appreciated in childhood, boldly coloured and strongly drawn, which the Reverend W. Awdry himself observed as being "accurate and consistent", even if they were emulating Dalby's work to some extent.
Co-operation also appeared to be high on Christopher's agenda, as he tried to provide new viewpoints and angles for Clive to illustrate to avoid making them overused and become dull and repetitive.
Spong seemed to evolve his style throughout his time as illustrator. First using solid and bold work to portray the scene to a more water-coloured style were engine's faces were more sand in colour than their normal grey. The further Spong went into the series, the further he went to making his own style in the Railway Series instead of emulating previous works.
After the Railway Series was put on hiatus in 1996, Clive primarily worked on illustrating educational books and did little in the way of fiction. However, in a late 2006 interview with Sodor Island Fansite, he stated he would return to the Railway Series if given the chance. He was as good as his word when the Railway Series returned in 2007 with Thomas and Victoria. And again, four years later, he was confirmed as the illustrator for the Thomas and his Friends.
Clive has illustrated many pop-up books and other Railway Series spin-offs such as Thomas and the Evil Diesel, Thomas's Christmas Party, Thomas and the Missing Christmas Tree, and a rewrite by the Reverend W. Awdry of Thomas Comes to Breakfast. He also provided illustrations for the BoCo story library book with Jerry Smith in 2008.
- Really Useful Engines
- James and the Diesel Engines
- Great Little Engines
- More About Thomas the Tank Engine
- Gordon the High-Speed Engine
- Toby, Trucks and Trouble
- Thomas and the Twins
- Jock the New Engine
- Thomas and the Great Railway Show
- Thomas Comes Home
- Henry and the Express
- Wilbert the Forest Engine
- Thomas and the Fat Controller's Engines
- New Little Engine
- Thomas and Victoria
- Thomas and his Friends
- Thomas and the Evil Diesel
- Thomas's Christmas Party
- Thomas and the Missing Christmas Tree
- Thomas Comes to Breakfast
- BoCo (with Jerry Smith)