So many choices...
Only one?? Okay... N&W #1218
It is already saved. Guess you didn't read the original poster's requirements?
Your idea of saved and mine differ somewhat. If it cannot run on demand, it needs saving. Probably, that is totally economically unrealistic.
I see thread after thread of projects to prep a locomotive to run a single excursion. IMO, that is a process of temporarily saving the locomotive, only to be set aside in a sorta suspended animation, hoping the elements don’t consume it before the next effort to temporarily save the locomotive.
My idea of saving a locomotive would be the Consolidation #1702 that has regular runs on the Great Smokey Mountain Railroad.
GSMR #1702 was originally built by Baldwin Locomotive Works in 1942 for the US Army. It was acquired by GSMR in 1994 and ran until 2004. Then in 2013 an agreement between GSMR and Swain County (NC) started the restoration (saving) of #1702. In the summer of 2016, #1702 was returned to service with regular runs.