The Coal Cart and Lid Serve as a means to house external electronics and batteries.  It should fit a 7.2V Cell 
I made provisions for (3) LED twinkle lights that you can send down stream to other carts.  Mock up shown.
I plan to just hit the buttons, but there is a remote too, I think I tossed mine out :/

There is a forward and rear facing hole for passing connections from the Coal Cart.
Lastly, I made a quick Lid for the standard cart
I plan to add a smoker to the Engine as well