Some countries allow it, e.g. Luxembourg has a policy. Hell, I'm even with my bike in a bus at the time of writing this comment.

The only catch is space... No fancy bike racks here or anything, just flexible space near mid entrance. You're at the mercy of other passengers, and for instance families with a buggy.

In my city all the buses have bike racks in the front. But could also be bikeshare.

It is a small finding bicycle that fits in a bag.