""Wind tunnel testing of a Prevost H5-60 articulated bus demonstrates a drag coefficient 50 percent smaller than that of a typical class 8 truck while maintaining similar cargo volume and exemplifying gap seals required for such streamlining."
Rail Versus Trucking: Who's The Greenest Freight Carrier? : TreeHugger
Would there be any point in attempting to streamline a train?
If you put an accordion between semi and trailer that was the width of both, would that be more aerodynamic?