Thomas is engine number 1 on the Fat Controller’s railway.
In the Railway Series and Thomas & Friends, Thomas works on the North Western Railway, the principal railway on Sodor controlled by Sir Topham Hatt, better known as the Fat Controller. His yellow-outlined number 1 is painted on both sides of his blue tank.
Thomas’s number reflects his central role in the franchise, but it does not mean he is the largest or most powerful engine. He begins as a station pilot, shunting coaches, and later earns more responsibility, including passenger work on his own branch line to Ffarquhar.
The other options identify different engines: James is number 5, Gordon is number 4, and Edward is number 2 in the classic Railway Series numbering. Numbers can vary for some characters across adaptations and railways, but Thomas’s status as the North Western Railway’s number 1 is the standard answer to this question.