Good Apps for travelers between cities in Europe especially

I'm not sure you'll find a single app that will get you everything you want.

The most comprehensive app is probably Google Maps. It will have most trains, many local public networks (but by far, not at all of them), and often also includes ride-sharing services (Uber and equivalents), as well as many flights.

But in many places the only option to get local transport info is the local network's app (if there is one) or their website.

Even for long-distance travel (especially trains), you'll usually need the national or regional operator's app or website to buy tickets (and sometime multiple apps).

If you have to travel complex train itineraries, special mention for the SBB app (and website), especially if you travel to/from/through France: SNCF apps are incapable of producing an itinerary with more than 2 stops, while the SBB app will give you those itineraries (you can then split it in 3-leg trips for separate booking).

Probably not the answer you are looking for, but here is what I'm doing. When I travel to Europe, I take A LOT of apps. The app that each individual supplier provides are (often) quite useful but they only cover the services of that specific provider. Last time we went in August, we brought (for a single trip!):

1. Airlines: United, Lufthansa, Swiss
2. Flight utilities: myTSA, Priority Pass, Loungebuddy
3. Trains: Deutsche Bahn (we only were in Germany and Switzerland)
4. Local public transportation: Berlin VBB, Zurich ZVV, Zurich "online ticket app:", bike rental Berlin (was awesome!)
5. Hotel: IHG, AirBnB, Intercity Hotel, Hilton
6. Ride Share: Uber (so-so coverage but still your best shot in Europe)
7. Cars: Sixt, Avis, Car2Go
8. Maps: Google Maps, Here We Go (great for offline coverage)
9. Attractions, restaurants, reviews: Google, TripAdvisor, Yelp
10. Other: Coronavirus Testing app, Proximity tracking

I typically don't do travel planning on the phone since the screen is too small to deal with the amount of information to be looked at. I have a whole set of travel planning tools that I use on my a laptop, but I don't think they would qualify as "apps".

So my recommendations would be to customize your apps to your specific itinerary. They are easy to download and easy to delete. On a longer trip we do "app maintenance" every 2 (or so) legs, whenever there is good WIFI and power for the laptop.