William Middleton was the original illustrator of the Railway Series.
William Middleton's illustrations for The Three Railway Engines were soon after replaced by C. Reginald Dalby's, which have gone on to become the more recognized and associated illustrations for the book. Middleton's illustrations were very simple, and little reflected any kind of real locomotive, making the characters look very much toy-like as opposed to the effect the Reverend W. Awdry desired. The reason for this was that Middleton was an inexperienced illustrator with little idea on scale or drawing people, who had been hired through Edmund Ward's printing connections.
The artist caused a whole heap of trouble for the future, including Henry's wheel arrangement, and two tunnels at Ballahoo instead of one. The illustrations provided by Middleton were replaced by reproductions by C. Reginald Dalby in 1949, with the originals being discarded and lost to time, consigned primarily to early editions of the book and being very rare to find afterward. However, the mistakes made by Middleton were picked up by Dalby, and so the problems continued onward.
Middleton was eventually fired by Awdry and he was subsequently replaced by Dalby as the illustrator of the Railway Series.
All biographical information was retrieved from the Sodor Island Fansite.