I'm going to argue with the Quizmaster about this. (I can do that - he fires for me). I've seen shots of 340 very dead in storage in Gunnison in the late 1930's. From the faded, peeling paint with old lettering showing through, it looks like it had been parked for quite a while. As Herb mentioned, it was the spare in Montrose from 1949-53 and I think it spent most of that time parked. Pictures of it 1952-3 show it with a nice but very faded green paint job. But even if you add all that dead time into the equation, it probably wins although RGS 41 is a close second.
K28's must hold the record for longest continous service of ANY locomotive (SG or NG) on it's original RR (not necessarilly original owner).