William Middleton was a lithographic artist, whom was known to Edmund Ward through their printing connection and was hired to illustrate The Three Railway Engines because the Rev. W. Awdry's illustrations were deemed inadequate for publication. Middleton drew his pictures onto stone for which he was paid a fee of £62. Unfortunately, Middleton was inexperienced as an illustrator and had little sense of scale and not much idea about how to draw people. As for the engines, each had a flat disk (looking suspiciously as if it had been traced around a coin) stuck on the front of the smokebox, on which faces had then been clumsily drawn. The illustrations were, overall, considered to be dull, poorly coloured and lacked subtlety of line.
The Rev. W. Awdry was very disappointed in the illustrations and he had emphasised through his agent that the engineering detail should be accurate. Middleton's illustrations ended up causing many problems, particularly in regards to Henry; In Awdry's original illustrations he had been drawn as a 4-4-2, however Middleton drew him as a 4-6-2 and virtually identical to Gordon, especially after his repaint at the end of the book. A more glaring continuity error was the way Henry's Tunnel was shown. 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, William Middleton was not acceptable as a permanent illustrator and Wilbert's agent insisted on picking an artist of their own choosing for Thomas the Tank Engine, which resulted in Reginald Payne being hired, whose illustrations were considered "immensely superior" to Middleton's by Awdry. Middleton was, however, 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 illustrating 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 more famous illustrations. While Dalby's illustrations still followed Middleton's designs and thus contained many of the same mistakes, they were considered a vast improvement.
Middleton had his studio on Halford Street in Leicester, above some shops, which he used from before the Second World War into the 1950s. According to the Rev. 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" and as a result, did not put much effort into his illustrations.