The Island of Sodor was intended to be fully canon, as a sourcebook for parents reading to children who wanted more detailed information about the world of the books in order to answer questions. It’s just more in-depth.
You can think of it as the Railway Series being the simplified ELI5 version, and TIS being the “real” version. For instance, the RS only mentions Sir Topham Hatt, without the clarification given in TIS that this relates to a father or his son depending on in which period of Sodor’s history the story is set.
The Island of Sodor was intended to be fully canon, as a sourcebook for parents reading to children who wanted more detailed information about the world of the books in order to answer questions. It’s just more in-depth.
You can think of it as the Railway Series being the simplified ELI5 version, and TIS being the “real” version. For instance, the RS only mentions Sir Topham Hatt, without the clarification given in TIS that this relates to a father or his son depending on in which period of Sodor’s history the story is set.