Bill said: The parts were interchangeable because of technology at the time. That's no longer true.
That's no longer true because there is no thought given continuing production nor to using existing tooling to make new stuff. It appears that every locomotive is designed independently of the rest of the line. The only tooling that is shared is the classic freight car tooling and passenger car tooling. As for having product left over, That's an investment in the future. It used to be that companies were used to having product "on hand." Now it's all about Just In Time manufacturing, which often becomes Never On Time.