Standard gauge rail still has several advantages: first, I imagine at least for the wheelsets and the bogeys they could use standard parts; second, even if you can't use standard train cars in the subway, you can use train tracks to move your cars - the Munich subway does this often, e.g. this article is about trains being shuttled several hundred kilometers to be retrofitted with a fire extinguishing system (https://www.u-bahn-muenchen.de/blog/2022/03/b-wagen-auf-gros... - of course you can't do that with light rail vehicles, the cars have to be built to comply with railway standards).