Thomas Tells a Lie, retitled Thomas Breaks a Promise in some versions, is a book.
Autumn is approaching the Island of Sodor, but Thomas complains that he has not been able to have any fun. Gordon claims that they are not there to have fun, but this only makes Thomas cross. Later, Sir Topham Hatt gathers his engines and informs them that a new branch line has opened. He needs a volunteer to check the signals, and Thomas says that he will. Sir Topham Hatt allows Thomas to check the signals and reminds him that safety is their first concern.
At first, Thomas does well with his job. The branch line had dangerous curves and hills, and lots of signals. Thomas is nearly done when he sees a sign for a carnival. Wanting to have fun, he decides to check the signals later. There, he sees the carnival, and the children cheer at the sight of Thomas.
Back at the sheds, Sir Topham Hatt tells Thomas that he has been gone for a long time; he must have done a good job checking the signals. Thomas says he has, but then realizes that he had forgotten to finish his job. With a happy Sir Topham Hatt, Thomas knows he cannot tell the truth now. He begins to worry; what if the unchecked signals didn't work? To resolve this, he decides to wake up early and check the rest of the signals. Little does he know Percy's mail run is diverted onto the branch line after the normal route is out of service.
Percy sees most of the signals, but does not see an unlit one until it is too late. With pouring rain, the slippery rails almost derail Percy. He does not, but one of his mail cars derails and break. The next morning, Thomas sneaks out of his shed when Percy pulls in with Sir Topham Hatt. Thomas apologizes, and confesses that he forgot to check the rest of the signals, what with the carnival and children and all. As a punishment, Sir Topham Hatt makes him check all the signals on Sodor - twice!
Now, Thomas always double-checks the signals to make sure they are working. Gordon still teases him about them, but Thomas does not mind; safety is their first concern.
- The book was featured as a read-along bonus feature on both, Percy's Chocolate Crunch and Other Thomas Adventures and Thomas Gets Bumped and Other Stories DVD, narrated by Robin Smith.
- When Thomas stopped at the red signal the points crossed over. They are supposed to join.
- Gordon is too heavy to go on branch lines.
- Toby's funnel is missing.
- James' splasher is not coloured black.
- Percy's coal bunker is green.
- Gordon's tender axle boxes are coloured blue.
- Percy's headlamp changes position.
- James' wheels and tender axle boxes are coloured grey.
- In the fifth illustration, Thomas' back buffers are missing.
- According to the text, Percy saw an unlit signal lamp. However, the illustrations do not show any lamps on the signals.
- In the illustration when the engines are gathered, Gordon and Henry's funnels are too tall.
- Percy lacks his crew when his second mail van breaks.