Best train app for Intercity, regional, and intercountry trains?

no matter which app you use it will be the same trains on the same tracks, mostly operated by different branches of DB.

Use the DB app - it is very good.

For the Austrian trains use the Oebb (Öbb) or if you are in the right place at the right time have a look at Westbahn. They have a most excellent webpage but no app that I know of.

I rely almost exclusively on DB for travel within Germany... but OEBB is useful as well whenever Austria is involved. I have no issue with schedules at either. Route maps appear with any DB itinerary you pull up. As for detailed railway maps for all of Germany... The network is incredibly dense. I don't know of a good one online. It's probably best to hunt down maps by individual "Land" (state.) Here are some covering southern Germany which I keep links for:

Bavaria

Baden-Württemberg

Rheinland-Pfalz u. Saarland

Hessen

And here's one I have for Austria, which I know to be incomplete.

Mark, does tge DB website show/sell tickets for the RegioJet and Westbahn trains?

Also on long-distance some connections are more affected than others.

And do you have a list of the reliable routes?

does tge DB website show/sell tickets for the RegioJet and Westbahn trains?

RegioJet is not active in Germany despite repeated declarations of intent. As far as Westbahn and Flixtrains are concerned, the DB website shows their trains but does not sell tickets for them.

And do you have a list of the reliable routes?

This would be pointless, because the railways are like the Hydra of Lerna: as soon as one construction site is closed, two new ones open elsewhere, often on very short notice by the network operator InfraGo. This makes life difficult not only for passengers but also for carriers. InfraGo claims that construction capacity is often only available at short notice, but its critics say that they simply wait until prices are at their lowest and don't care about the consequences of their actions. The only thing you can do is find out about the construction site situation before you travel.

And do you have a list of the reliable routes?

Fritz, that was in response to Mark’s comments above. It doesn’t matter if some of the routes are reliable if you don’t know which routes those are; you just have to assume they will all be unreliable.

RegioJet is not active in Germany despite repeated declarations of
intent. As far as Westbahn and Flixtrains are concerned, the DB
website shows their trains but does not sell tickets for them.

I asked because DB does list and sell tickets outside of Germany (I can buy a Vienna to Budapest ticket on DB for instance) and the OP was looking for something like The Trainline to be able to see all of the options in one place. Or that was my perception of the question.

Mark, for all that information, dont you have to pay for a membership?