Its important to realize that a loco that is "running" might not be working. If it had a main generator problem, it might not have been able to produce much power. It might be able to move itself, but maybe not more than that. if it had a prime mover problem, it might have been able to idle, but not load up (or spew oil out of every part of it when under a load)
Sometimes it is a bunch of things that finally put the nail in the coffen on the life of a loco - same as with you and your personal car. You can get rid of it when you get tired of it, or you can keep it until it "nickel and dimes" you to death, or somewhere in between. It seems that the 206 was at that point of it nickel and diming them to death, they kept it running as long as they could, with parts off other locos, and when the newer power came after the rebuilding of Sparta Mtn and the stack trains, there was no need to keep dealing with it.
As railfans we might look at the locos, sometimes, particular ones, with admiring eyes, but in the end, they are still just a big piece of machinery, that can wear out and needs maintence and work and has a life cycle, just like everything else that surrounds us.
but this is a very interesting discussion on the end of life of the loco in revenue service.
On the RR, "believe nothing you hear and only half of what you see"
John, aka "JTGSHU" passed away on August 26, 2013. We honor his memory and his devotion to railroading at railroad.net.