Ur byþ anmod and oferhyrned, felafrecne deor feohteþ mid hornum, mære morstapa þæt is modig wuht
10:
Because they were never called that and Thomas is just one character. The first book was called The Three Railway Engines and did not even feature Thomas, who only appeared in book 2 of The Railway Series. Moreover, most of the stories do not feature Thomas, and many of them feature engines that Thomas has never even met, let alone made friends with. All the books have borne the series title The Railway Series.