One track of the 3 is for wheel truing only.
Inspections: they take priority. They must be done on the day specified. If there is a defect that can't be repaired that day, it has to wait to go back in so the next group of cars can be inspected the next day, but in the overwhelming majority of cases, the inspected cars would be good to go out the next day for service. The backlog of "trouble cars" have to wait. But in the case of the Concourse R68's the troubles are repaired pretty quickly since the D does not have a large spare factor. Even if they had more spares, more cars would have to wait their turn to get in the barn. And that statement goes for every barn. As long as the D makes full service every rush hour, it's all good.