The original 42 books are the only Thomas I read. That and the one about how Bulgy becomes a vegetable bus. There are lots of good lessons in them and also lots of good vocabulary (some parents shy away b/c of the advance vocab but to me it is a plus).

In the original series the backstory is the decline of the steam age, and Sir Topham Hat's efforts to more or less create a heritage railway to save the steam engines he loves from scrap.