There are a couple of reasons why the consist sometimes ends up this way. For one, we have a limited number of operable freight cars and many of these have 3/4 size couplers. The hoppers Dan mentions have a full size coupler on one end of two of the cars. These must be operated as a set or coupled to another 3/4 car, such as the steel flats. This quickly becomes a nightmare when putting a train togather. We have one emergency knuckle modified to fit a 3/4 coupler, but that only solves one compatability problem.
The other reason is that we try to put togather a different train each time we do this, and that becomes problematic for the above reason. Over time (and with funding) we will convert more cars to full size couplers. We will also restore more original SV cars, but as many of these are one of a kind that will still not help much.
Anyway, an authentic Sumpter Valley freight train would consist of (for us) a long string of empty flats with a couple of box cars and possibly a tank car or day coach thrown in; not a vary photogenic train. We hope to recover the 4 SVRy passenger cars from the White Pass and these, with some restoration, would make an authentic passenger train.