William Middleton was a lithographer who was known to Edmund Ward through their printing connection. Middleton was paid £62 to illustrate The Three Railway Engines after the Rev. W. Awdry's illustrations were deemed inadequate for publication. However, Middleton was inexperienced as an illustrator, had little sense of scale, and was clueless when it came to drawing people. The engines in his pictures each had a flat disk (giving the appearance of having been drawn by tracing around a coin) stuck on the front of the smokebox, on which faces had then been clumsily drawn.  The illustrations were considered dull, poorly coloured and wanting in line subtlety.
The Rev. W. Awdry was severely disappointed in the illustrations, having emphasised through his agent that the engineering details needed accuracy. Middleton's illustrations ended up causing many problems, particularly in regards to Henry: whereas in Awdry's original illustrations Henry had been drawn as a 4-4-2, Middleton drew him as a 4-6-2 and virtually identical to Gordon, a problem exacerbated by Henry's repaint at the end of the book. A more glaring continuity error was in the depiction of Henry's Tunnel. The text stated that the tunnel had only one bore prior to Henry getting stuck in the tunnel, with a second being constructed afterwards. Middleton, however, illustrated the tunnel as having two bores from the start.
Due to the poor standards of his illustrations, Middleton was not invited to illustrate subsequent books and Awdry's agent insisted on picking an artist of his own choosing for Thomas the Tank Engine, which resulted in the hiring of Reginald Payne, whose illustrations Awdry considered "immensely superior" to Middleton's. Regardless of his disinvitation from illustrating The Railway Series, Middleton was employed to design a silhouette of Thomas and his coaches which was gold blocked onto the cloth bindings of the early editions of the books. 
Middleton was not credited for his work on The Three Railway Engines, and his involvement was not widely known until the publication of The Thomas the Tank Engine Man in 1995. In 1949, for the fourth edition of the book, C. Reginald Dalby was commissioned to re-illustrate The Three Railway Engines, which have since become the book's iconic illustrations. Although Dalby's illustrations retained Middleton's designs and thus shared many of Middleton's errors, they were considered a vast improvement. 
Middleton's studio was above some shops on Halford Street in Leicester, which he used before the Second World War and into the 1950s. According to the Reverend T. Robin Martin, who knew Middleton through his grandfather, Middleton did not think The Railway Series would be a success as it was about "dirty old locomotives." As a result, he did not put much effort into his drawings.
- William Middleton on Sodor Island Fansite