Working in the industry: this is a can of worms that you might regret opening, because of simply how completely insane so many of the systems there are. I've seen cities repurpose text fields in otherwise perfectly standard formats because they needed a way to indicate that the bus was {insert any absolutely insane situation}, and not parsing that field means that the data doesn't make sense.
Your best bet for most of Europe are the open data platforms. Example for France: https://transport.data.gouv.fr/. There's soooome standardisation around a few formats:
GTFS, Netex, SIRI along with their various extensions (like GTFS-RT for realtime data), etc. Just parsing these (which is already a large undertaking in and of itself) should get you covered for a bunch of networks.
Oh, also, much of the data you'll find, especially from smaller cities or regions, is awful. You're going to be told that the line icon is white, on white text, and that's actually perfectly normal because actually their bus header is from an obscure system from former Yugoslavia that actually interprets "0xFFFFFF" as black when on layer 1. Good luck!