An anthropomorphised fictional steam locomotive in The Railway Series books by the Reverend Wilbert Awdry and his son, Christopher, published from 1945.

A little blue tank engine who works on the North Western Railway.

He originally worked as the station pilot at the Big Station but longed to leave his yard to go out and see the world.

After helping to rescue James from a nasty accident, Thomas was rewarded with two new coaches, Annie and Clarabel, and was given the responsibility of running the Ffarquhar Branch Line, where he works to this day.

When Awdry created Thomas, the engine existed only as a wooden toy made for his son Christopher. This engine looked rather different from the character in the books and television series and carried the letters NW on its side tanks, which stood for "No Where" according to Awdry.

Thomas wasn't originally based on a prototype; rather, the initial stories were an accompaniment to the toy made for Christopher. After Awdry's wife encouraged him to publish the stories, the publisher of the second book in The Railway Series, Thomas the Tank Engine, hired an illustrator named Reginald Payne.

Awdry selected a real locomotive for Payne to work from to create authenticity: a Billinton designed 0-6-0 E2 Class of the London, Brighton and South Coast Railway. This may have been chosen simply because Awdry had a photograph to hand.

Thomas as portrayed by Hornby and the TV series is based on one of six E2 class locomotives fitted with an extension to the front of the water tanks. Awdry was unsatisfied with one detail of the illustration — the fact that the front end of his footplate featured a downward slope, which meant that his front and back buffers were at different levels.

This was an illustrator's mistake that was perpetuated in subsequent books. The accident, in "Thomas Comes to Breakfast" was partly devised as a means of correcting this. Thomas has always been shown with a curved front buffer beam in the television series.

Payne was not credited for his illustrations at the time, and it is only since the publication of Brian Sibley's The Thomas the Tank Engine Man that he has started to receive major recognition. It had often been erroneously assumed that C. Reginald Dalby created the character, as he was responsible for illustrating books 3–11 and repainting the illustrations of books 1 and 2.

Although Thomas locomotives are seen today on various heritage railways, the locomotives are either unpowered decoys or converted from other locomotives, as all of the prototype LB&SCR E2 class locomotives were scrapped between 1961 and 1963. A project has been proposed to build a new one.

Thomas is described as being a cheeky and fussy engine. He often gets into scrapes, usually by being over-eager to do things best left to bigger and more sensible engines. But clouds never last long in Thomas' life and he is soon bustling about again, playing his part in the yard and on his very own branch line, of which he is extremely proud.

He loves teasing the others, especially the bigger engines, such as Gordon and on occasion brags about his superiority, but is always brought down to earth in due course by anyone willing to correct him, especially the Fat Controller and his two coaches, Annie and Clarabel. If Thomas has one major character flaw, it is that he is forgetful and rather impatient. However, he is also optimistic, idealistic and altruistic with a heart of gold.

A friend to all engines and a popular member of the Fat Controller’s Railway, Thomas is No.1 and does his best every day to live up to that through helping his friends and those that he cares about. He lives to be a Really Useful Engine and encourages everyone else to do the same. He likes to be better than James and Bertie in races. He can, however, also get annoyed about being useful such as the time when Rosie followed him around because she idolized him so much.

While he might drive others away, Thomas is very good at luring back friends should the situation demand it. Despite all this, he does have his pride and will sometimes take wrong advice at the wrong time - such as from Sailor John or Ace, as well as refusing help even when he needs it.

In other cases, Thomas can be highly curious and positive, with slight hints of skepticism and forgetfulness, including tendencies to daydream. He is also however depicted with a subtle tone of wisdom, helping engines like Sonny and Rajiv be really useful, despite their difficulties or insecurities in the past.

Thomas also has a high perspective on odd objects or people, quickly growing suspicious upon seeing the Tiger Hunters' hidden cage, Diesel 10's plans to take over the railway of steam engines with his fellow diesels and Baz and Bernie's whispering.