The cars were picked by a railfan's selection procress to get the good cars that were good examples of the type. All the M of W cars were taken but the box cars were switched out to get the good usable ones for the passenger cars. Anything special by type or age was selected.
Maybe others can add to this reply. Hope that helps. I can only guess that Terry Ross was directly involved in the selection, as he was the leader at that time. I must note he was not too skilled on keeping water up in the boiler of #483.