I would ask this as LOVE...
And I will admit it is ME of 48 who still loves them.
My son, now 7, barely likes them most days, but WE both enjoyed them together when he was a little guy
When he was just crawling he only had a few, but he could tell them apart and could say their names...Thomas, Percy, Bertie
They were very durable, and as a learning tool, with so many characters and colors, faces and designs, I chose them as a fun way to learn...names, colors, facial recognition, vocabulary, imagination, general play, and on and on...
He loved Thomas!!
Then one day I decided to "Collect them All"...long story, but over the journey of the past 6 years, it is I who is left in love with them...
It IS the faces that they ALL have, and the structure of the series that I appreciate.. They all smile.
They are all "known" as the characters that they are, by all of their character flaws being clearly presented, understood, and accepted as themselves.
ie: James is smug most often than not, uncoincidentally James is one my least favorite character...lol
SO FUNNY...I REALLY wish all trains had faces.