Thomas the Tank Engine first appeared in 1946 in the book Thomas the Tank Engine as a station pilot, whose job was to shunt coaches and trucks for the bigger engines. He longed for more important jobs such as pulling the express train like Gordon, but his inexperience prevented this. Eventually he was responsible for rescuing James after an accident, and the Fat Controller (then known as the Fat Director) decided that he was a Really Useful Engine, and ready for his own branch line. He has remained in charge of this line ever since.

His closest friends are Annie and Clarabel, his two coaches. He is also very good friends with Percy (despite a lot of arguments), Toby, his old friend Edward, and his frequent substitute Duck.

Thomas is based on the E2 Class 0-6-0T locomotives built for the London, Brighton and South Coast Railway between 1913 and 1916.