It seems that historic engine preservation suffers from the same sort of malady as historic aircraft. Every potential pilot/engineer wants to fly/run the last "whatever." The Air Force museum had to adopt a policy of disabling every aircraft it owned, to ensur that all those pilots don't take the last whatever up and crash it.
The locomotives are also a finite object. Take that last one out and wear it out, then you'll have to replace every part till it isn't historic anymore. Especially once you've used up all the historic replacement parts.