they don't. cars are assinged to the yards and the yards are picked by their capcity to care for the new cars. Corona didn't get any R142/As becuase they had no way to get to the roof mounted HVAC units until the new shop opened. 207th can't fit a 300 foot long unit on it's insepction pits, which is why the 60' SMEEs have survived there so long. The execption to this is the R160A-1 cars which were made for the eastern Division so that the MK R42s could be used to replaced older cars elsewhere.
when it comes to putting cars in service, it's based on the crew who hae been trained to use the cars and what lines those employees have picked to work.