i got some items

that could be one but its also the fact that the windows on the side aren't painted and there's no door detail like most models of Thomas 
look at this regular trackmaster Thomas 

and then look at Thomas in a jam the side doesn't have that door detail like the tomy/trackmaster one 

but that's not all the prototype has the door detail or is it shadows but the final one does not have them. I think they accidentally removed the door shadow. I may be wrong