Rev. W. Awdry 
Thomas Breaks a Promise (Thomas & Friends) [EPUB ebook] 

Thomas promised Sir Topham Hatt that he would carefully check all the signals on the new branch line. But when he is distracted by a carnival, he forgets all about the signals. That night, Percy has a close call on the new line when one of the signal lights is out. Will Thomas admit that he forgot to finish the job?From the Hardcover edition.