brokenrail - you are correct in that the -primary- Columbus-Indy freight route changed over to Union City (in early 1973, I think).
I believe that in PRR days, a Richmond crew ran a daily Dayton Turn to move western traffic to and from the Dayton-Xenia area. The cars were then plugged into the mainline trains at Richmond. If traffic was down, it moved to Cincy or Columbus in regular freights - but this called for extra handling and at least a day's delay.
The move to Union City dried up the Richmond connections - the cars now had to get to and from Avon Yard in Indianapolis to make western connections. This is why PC negotiated for the "through crews" and set up two pairs of through freights via Dayton. I had understood (corrections appreciated if needed!) these were bid runs, with the crews getting paid whether the schedules ran or not. As a result, if traffic warranted only one train, PC would run an overhead train via Dayton to avoid wasting a crew. The Trailvans were popular candidates for this treatment, as they could run Indy-Columbus with ease, would not have problems with the one-setout/pickup-per-trip rule, and handle Clement Hill without helpers. In fact, I think that at some point this became the preferred routing for at least one of the TV schedules.