Durango and Silverton museum employee here- my understanding is that 486 was always a bit too temperamental for the engine crew's liking. Russell Heerdt told me that when he fired her back in 2017 she had a really violent rock going and actually shook the engineer's seat to pieces during the run. Any D&S folks with firsthand knowledge of why can feel free to correct me if I've misunderstood why she's been chosen to stay in the museum. I'm are pretty happy to keep her inside, with 478 getting pulled from the museum to be converted, poor old 42 might get lonely without another locomotive to keep her company.