Aerodynamics?
 

https://lh5.ggpht.com/_4xQFBZJE2fI/S8...0/DSC04809.JPG
This is something relatively new. What are the tabs on each car for?

Could they be to keep people, animals, things from squeezing in between yet allow the cars to articulate?

No. There is a very heavy rubber tube on each car surrounding the door and when the cars are joined nothing passes through.

Guess you could write the company and ask them? ;)

I would also guess that they'd be for keeping objects form getting pinched in there.