That’s not exactly right; the layer tag is relational: what’s on a bridge has a layer higher than what’s under it, which can be 0 / -1 or -1 / 0 or even -4000 / 12345. In this case it was much simpler to change the layer value of the streets under it than to cut all the railroads to set a layer on them. See https://www.openstreetmap.org/note/4739742 for the context.