Thomas & Friends is a British children's television series which aired from 9 October 1984 to 20 January 2021. Based on The Railway Series books by Wilbert Awdry and his son Christopher, the series was developed for television by Britt Allcroft. The series centers on various anthropomorphic steam locomotives as well as other vehicles living on the fictional Island of Sodor. The show was initially filmed in live action on model sets, whereas the latter half of its run was produced using CGI. Over 500 episodes were produced over the course of 24 series.

In the United States, it was first broadcast along with the spin-off series, Shining Time Station, on PBS' PTV Park block on 29 January 1989, while broadcast of the series did shift over time, it later aired on PBS Kids up until 2017. The rights to the series are currently owned by HIT Entertainment (a subsidiary of Mattel), which acquired Gullane Entertainment in July 2002. HIT was folded into Mattel in 2016.

An American 2D animated reboot, Thomas & Friends: All Engines Go, premiered on 13 September 2021 on Cartoon Network's preschool block Cartoonito, ending its run on 11 September 2025 with the final set of episodes released on Netflix.

The first attempt to adapt Awdry's stories for television came in 1953, when the editor of the Railway Series books, Eric Marriott, was approached by the BBC, who wished to use live-action model trains to re-create two stories from Awdry's first book, The Three Railway Engines. 00 gauge Hornby Dublo models appeared on sets that reflected the style of the original illustrations. The first episode (based on "The Sad Story of Henry") was broadcast live on the evening of Sunday 14 June 1953 from Lime Grove Studios. The live broadcast did not fare well: a failure to switch the points caused the model of Henry to derail and it had to be replaced on the rails by one of the operators. The models moved jerkily, and all effects and music had to be superimposed.

News of the broadcast hit the front pages of The Daily Telegraph and the Daily Mail. Awdry branded the episode "unprofessional", and the point-switching debacle an "elementary mistake". As a result, the second episode scheduled for 28 June 1953 was put on hold, and then later cancelled. The BBC offered Awdry and the Railway Series publishers greater creative control over the production, but the publishers declined, preferring to focus on publishing new books for the series.

Nearly 20 years later, the BBC featured Awdry's stories in the television story-telling show Jackanory. Fourteen years before Thomas and Friends was aired, Ted Ray (sitting in a stationmaster's office) read five Railway Series books in episodes that aired between 20 September to 2 October 1970.

In 1973, Andrew Lloyd Webber, having read The Railway Series as a child, approached publisher Kaye & Ward with a proposal for a musical television series, with songs from himself and lyricist Peter Reeves. However, the publishers and the author refused to give Lloyd Webber's company "control of almost everything", which Lloyd Webber's lawyers argued was necessary in order to "secure the investment money from America which would be needed to pay for the animation and the film-making." The status of the project seemed uncertain, and while Stanley Pickard (Kaye & Ward's managing director at the time) told Awdry that he was "maintaining personal contact with Andrew and still had a slight hope that there might be a way out", Awdry remained apprehensive, saying that "Once the Americans get hold of it the whole series would be vulgarised and ruined." Eventually, an agreement was reached, and Awdry received an advanced payment of £500.

A television pilot was commissioned from Granada, which would feature 2D cutouts of the engines moving along a background in a style reminiscent of Ivor the Engine, with involvement from animator Brian Cosgrove. The cutouts and backgrounds were based on illustrations from The Railway Series. The pilot episode was completed by early 1976, but Granada ultimately decided not to produce a full series, as they feared that at the time Awdry's stories were not popular enough outside the UK to justify the time and money needed to make the series. Andrew Lloyd Webber later established the Really Useful Group in 1977, a name derived from the phrase "Really Useful Engine". He would go on to work on a musical loosely inspired by The Railway Series, called Starlight Express, which premiered in 1984, and became one of his most well-known works.