It depends what you mean.
When the first book in the series appeared (I thought it was 1948, not '45, but it doesn't really matter) there was no concept of an "Island of Sodor". Like many model railroaders, Rev Awdry developed a fictitional life situation to support his developing character set and story line. So it depends at what point in the development of the series your question refers to. But it also depends on whatever interpretation Christopher Awdry wants to put on his father's stories. (Did RW introduce Neil, or did Christopher? I can't remember). In addition, many of the later characters, especially the narrow-gauge ones, are based on entire existing lines. Some of the ones I can remember off the top of my head are the Ravenglass and Eskdale, Ffestiniog, Tallyllyn, and Snowdon Mountain. And sometimes there is no indication that the stories involving these characters are actually based on Sodor. And what if, at some future stage, Christopher Awdry introduces a storyline that says Neil did not actually arrive on Sodor until he was very much second-hand? That is, just because he's the oldest engine doesn't mean that he was actually the "first on Sodor".
I think the only person who can give a definitive answer to your question is Christopher Awdry himself. And having said that, he's quite at liberty to change things at any time in the future. He might decide to write a whole bunch of pre-quels!