I tried making the material zones all animated to go invisble. I tried turning off specular value for any node, IF it was available. Where it had alternate specular, I animated that to zero. In the above image, the only node connected has no specular value so i animated texture strength to zero, which got rid of the texture, but didn't make it any less visible. I tried a lot of things, but not understanding doesn't help.

Failing all that, I did "sort of" do what Little Dragon suggested. I rendered the background, foreground, and then created the bus in PS, making frames where it was less and less opaque until it disappeared, then layered the images and voila, disappearing bus. Not optimal, but it worked.

Had to mess with the sandwiching because I was using volumetric smoke effect. Had to dump that effect because i couldn't figure out how to layer that effectively.

The bus is a freebie from sharecg with many parts and nodes. Great bus, too many parts. ;) Have to see if I can stand how it looks to see if I want to try to make it go invisible the correct way. Why can't anything be simple? Just a dial for Transparent? hmmmm? Gah!!!

Thanks everyone.