Matt on most of the express type passenger trains they would run a normal sitting car like a FS between the aircon cars, and the guards van, which were first class for the second class passengers.
This also happen on the express's with sleeping cars they would often have a second class sitting car on the end.
Even the Indian Pacific back in the old days would have a few sitting cars on the end, these were for sitting passenger who were only travelling on the NSW part of the trip - they were added and removed at the hill. You could not book on the IP part of the train for travel withing NSW. You could Join the train but only for interstate travel. I often travelled in the siting cars between Bathurst and Sydney.
The IP also often had MHO's or the KB mail vans - an MHO with out center doors added to rear of the train for the Sydney - Broken Hill part of the trip