Stories about a narrow gauge line (Sodor's equivalent of the Snowdon Mountain Railway?) that connects with the line on which Thomas and friends run. There are indeed four engines serving this line, although only three of them appear in the stories.

This doesn't manage, as the best children's books do, to entertain the adults well; a problem compunded by an author's note that confuses the names of human and mechancial characters.

For release, hopefully in a railway setting.

Not a narrow gauge railway, nor a tourist line, but the carriages on the deep tube lines must be almost as small as those in this story. Although they are the bogie coaches that Sir Handel wanted...

We got on a train at Hatton Cross, and left the book on the window ledge. It was still there when we got off at Green Park.

I remember these books from my childhood days.