The Twin Lambs - 1931

Marjorie Flack (1897-1958) was the author of The Story About Ping! We met the illustrator a couple of days ago.

Helen Fuller Orton (1872-1955) was a prolific writer and mostly wrote books whose titles started "Mystery of the__" The Little Lost Pigs was another of her books, published in 1925. The Twin Lambs is available used, but I could not find a cover picture.
Stokes publishing had some prime ad space down the side of the page:

  

I've never seen any pigs riding elevated trains; but I have heard of  This Monorail.