I think I've gone and made the boards .25" wide, so I can accomplish a 1" and 1.5" grid by simply lining up the seams. If 1" == 5', then that means each board width represents... 15"... That's not TOO wide, so I'll leave it as is.
I will try to make some metal bands around the edges to tie it all together. I'd like to also make a metal minecart texture, if only so I can practice making a metal texture.
I'll darken these wooden boards and make a buffer box, too. I think the buffer box will be 1"x1.5"x.5", so it'll be smaller than the mine cart.
For the mine carts and the buffer box, I'll make it so it's a textured, empty, lidless box. Then I'll make some simple inserts for it - dirt/rocks (for the buffer box) and dirt/rocks/coal for the mine cart.