Well, surely whom they really use is an "expediter" or "freight forwarder", like the company whose website is shown in the original post (other terms/names are used, too). Railroads are close to natural monopolies - there are only a small number of examples where more than one location is served by more than one rail line - and so to get from continental Point A to B naturally can necessitate using Railroad A, B & C. The expediter does the organization of making the appropriate contracts; when the transportation event occurs, these days normally the entire railcar remains intact and simply is pulled by the different railroad's locomotives across the various sets of lines. That is why you'll see, for example, a Burlington Northern/SP car being pulled by a Chesapeake train, and so forth.
