The first 26 books represent the "Wilbert Awdry Era," characterized by anthropomorphic locomotives and stories often based on real-life railway events. Rev. W. Awdry. Illustrators:
While the original individual books are often out of print, they are frequently released in comprehensive collections. Amazon.com The Railway Series Books 1-26 Pdf
Including William Middleton, Reginald Payne, C. Reginald Dalby, John T. Kenney, and Peter and Gunvor Edwards. The first 26 books represent the "Wilbert Awdry