> Also, what's your source for the import?

This is not an import. The source is the OSM data itself. No external data, other than Wikidata items, was added here. I did cross-reference with a GIS dataset produced by the FRA (Federal Railroad Administration), though OSM data already largely matched the dataset so nothing was changed in that regard.

> as far as I'm aware it's still referred to as "Union Pacific Valley Subdivision."

By whom? Certainly not the FRA, Caltrans, or Union Pacific. The Valley Subdivision here was originally tagged with the operator in the name to disambiguate with the Valley Subdivision on the other side of the state operated by SCRRA. But in general, including the operator in railway line names is a discouraged practice—the operator belongs in the operator tag.