The 'normal' system being complex is a bit exaggerated here - if you have to travel one-off via local multi-mode transportation you'd usually just get a regional Deutsche Bahn day ticket which will generally by accepted by local transport companies as well, and I believe various variants of these would also exist for longer-timespan tickets.

Still, that would not be as handy as the mostly unified billing system using NFC cards we have in the Netherlands since the late 2000s, where if you do not have any entitlement valid on a particular mode of transportation, it uses a charge card balance as such.

While this isn't as innovative nowadays as it used to be, it's still notable as it works in the entire country, which, while small, is still not a city-state.