I think this box set is great (I like it so much better than the Thomas stories based on the tv shows), but in my opinion (and my toddler's) Blue Train, Green Train steals the show as the star in this collection. It is much catchier and more fun as a result than the 3 other books (not to say that the other ones aren't good, of course, just that we really love Blue Train, Green Train). It may be a matter of personal taste, but Blue Train, Green Train seems to have better rhymes and rhythm than the other books. I gave it four stars because I wish I liked the other 3 books as much as Blue Train, Green Train--but this might not be fair for the set as Blue Train is an outstanding book in my opinion.
This set is overall a really good value. The board books are on the smaller side (not sure of the exact dimensions, but they are the same size as the other Dr. Suess board books if you have seen those)--I think they the perfect size for a toddler to hold and flip through. The board books are great quality in both physical materials/design/sturdiness and writing (and the case is absolutely adorable) so it is worth checking out. But if money is tight just buy a used copy of Blue Train, Green Train.