I don't know if you already know #transitous but if you don't it is worth to have a look at it. It's some kind of routing system #freesoftware based mainly on open protocols as well as some APIs.
It combines every kind of public transport to route, sometimes you need to check how to write start and destination, but it is anyway amazing. You can check it on https://transitous.org/
Its library or API is also used by other apps I also use such as #Ktrip and #Kitinerary, or others like GnomeMaps