Note that your system would probably work equally as well without a shield (I'm assuming the bus has diesel engine as it has no ignition system to cause interference). RS485 is quite tolerant and in your application you could wire with hookup wire and no termination and it would most likely work - not that I suggest this!!!
In my buses the cables run in a cavity just above a row of fluorescent tubes with associated drives. Sometimes I think I would rather have a gas engine and tungsten lamps - oh well, dreams.

Erik