I was recently recommended https://api.transitous.org (it's a webapp despite the domain). It can do things like "someone can pick me up on the far side and I don't mind walking 30 minutes at the start at 6 km/h, what's the optimal route to take?"

Revolutionary! Finally I don't have to look on a map to find all stations in walking/cycling/driving distance and then route to/from every possible combination of start and end station manually to see what busses and trains run there. And repeat the process with each schedule change in each involved country. Every other transport app that I know of will try to keep you within a few hundred metres of a bus stop or train station at all times and say "there exists no route" before telling you to cycle 10 minutes or from which station you have the shortest remaining distance by taxi or whatever

KDE Itinerary, a mobile and desktop application, is becoming better and better for this. It's based on MOTIS / Transitous.

I just discovered cartes.app can do this too!

I've found that websites specific to a single European country or, in the case of France, to a single city are the ones that work best.

rome2rio.com works reasonably well to at least get an idea where to look up the exact prices and timetables