I just looked it up to find out for myself. It can be both 'to' and 'after', but there's lots more to it if you're still interested: https://yourdailygerman.com/2014/03/04/nach-meaning/
REGARDING: "Wo ist der Bus nach Frankfurt?" The correct answer from Duo was " Where is the bus for Frankfurt?" I got my answer wrong when I answered, " Where is the bus to Frankfurt?" In another question asking for a translation of the dialogue : "Wo ist der Bus nach Frankfurt?" gives the translation below as : " Where is the bus to Frankfurt?" Is Duo confused?