Yeah, since the book cover for the new special was revealed, I decided to share my thoughts on who might be these characters are in the special.
On the back, we see some sort of a steam shovel vehicle. I believe it could be Ned since he's the only one on Sodor. Besides, we got it correct last year that Jack was coming back in the show.
As of the new character, Henrietta. She is either that big green engine or the blue tank engine that looks like Millie. (Why, Brenner?)