--- Frank Greene wrote:

Are you saying the home road's cars earned the same PD on foreign
roads as the foreign roads' cars earned on the home road?

Yes, until differential PD came into being (after the STMFC era).
But home road cars paid no PD on their own lines, and that provided
some incentive to prefer to load foreign cars and get them off line
with a revenue earning movement. Also if one's own cars were better
quality than the foreign cars (e.g. equipped with roller bearings),
one might prefer not to send them offline if another road's car
was available for the load. At certain times of year a railroad
might hoard its own equipment online, as the GN did for the wheat
rush. This meant parking equipment for a month or more in
anticipation of the rush, and they didn't want to pay PD on those
idle cars.

Tim O'Connor