Currently OpenStreetMap routing API, would have to be replaced with the Google Maps Transit API.

Would be great to offer this as an option, even if we would have to fill in our own API key. The train tracks in my country differ quite somewhat from the roads when I try it out. Also. busses take defined routes and not the shortest path. Google Maps provides all these routes.

OpenStreetMap routing API should provide routing with Car, Bicycle and Foot profiles. You might have to modify the API call to change the profile.

PS: There might be other free alternatives to using the main osm.org API which will be more reliable.

May I suggest creating an option to add waypoints which should help recreate bus routes?

Would you get significantly different results? I'd expect this would work OK even if you assumed travel in a completely straight line - unless you're tacking like a sailboat, you're going to spend most of your time travelling close to the overall direction of travel.

At least i do. Tried my regular commute, but it has a bridge witch is dedicated for buses and emergency cars. So would be a nice addition, but the added complexity of using something like travel api in google maps, witch would cost more. Really liked the idea, and usually seems to work ok.