westnordost
Regarding which standard I am referring to: Basically the wiki definition of railway=tram (See the linked wiki page).

I know it's British English, but the important part about railway=tram is that it is a railway line that runs on the street. As far as I know, en-GB tram === en-US streetcar, there is no slightly different meaning, it is exactly the same.

So anyway, railway=light_rail does not have this implicit meaning. Furthermore, the wiki page for light_rail actually mention as first point in the definition of what "light rail" is: "neither tram nor rail".

You mention there is a difference in tram systems in use in SF (so I guess the historic line is railway=tram and the muni metro is light rail?). If they are incompatible to each other because of different electrification or different gauge width, can't this difference be described more accurately using the destined tags for those properties?