It is my understanding the drifting throttle was left open on 485. This from Salida guys I know. The engine watchman was a little guy who when he saw the 485 in the pit started jumping up and down and screaming "the son-of-b---- took off all by herself.
Management was not happy at all to lose the engine but decided not to repair her once the damage was evaluated. The boiler had too much damage.
parts from the 485 can be found on all the k36s. In fact parts from all the engines were traded with each other including tenders. If they had a good engine and a bad tender but a good tender sitting around from another engine, no way would they let tonnage sit around. And so it was with all parts. They did what it took to move trains.