Stew, I have no idea what the car is, nor I suspect did Tony Koester when he shot the photo. Probably storage mail in a common foreign line box car, but that's just a guess. It appears to be a summer to late summer shot, and there was probably no need for steam back to the coaches. If there had been, they would have no doubt tagged the box car on to the rear end of the train. Whatever the service required. Sorry we don't have more information for you. / Ron