Traveling domestically in Germany is very easy! For this post I will compare 2 of the most popular methods (rail and air), and tell you the clear winner, while comparing them in the same domestic journey (Bremen-Frankfurt).
Bremen-Frankfurt: Rail and Air
I travelled this route first via rail, then via air, and here are my thoughts:
First of all, my direct train took me from Bremen to Frankfurt in 4 hours and 10 minutes. I arrived to Bremen HBF (which is located in the immediate city centre) 15 minutes before the departure, and it dropped me off in the city centre of Frankfurt. On the way, I treated myself with a small coffee from the board bistro, and had the chance to observe the scenery, and felt no stress whatsoever.
When flying, the airline recommends arriving to the airport 2 hours before departure, as check in closes an hour before. From Bremen city centre, it took me roughly 25 minutes to get to the airport. Then I waited for 2 hours, first in line to drop my bags, then in the security line, then in front of the gate. Afterwards, the flight took roughly 45 minutes. In Frankfurt Airport, it took me another 20 minutes of walking to get from the gate to baggage claim, and then waited for a bit at the baggage claim. Lastly, I had to get to Frankfurt from the airport to meet with my friend during my layover, which took another 20 minutes on regional rail. So everything added up to roughly 3 hours and 50 minutes, and I was stressed for the majority of the time.
Rail: the Clear Winner
Even though the time you spend travelling is significantly shorter when you fly, the entire process adds up, and the time it saves is minimal, almost neglectable. Moreover, using the train is significantly less stressful (at least for me), and far more comfortable, even in second class.
Another important point to keep in mind is that if you book 2 weeks ahead, the train could cost as low as 19 EUR, meanwhile the flight will certainly be in 3 digits.
Beyond these points, it is important to remember that flying such short distances is extremely unnecessary because the carbon footprint of such a short journey is incomparably higher than the electric powered trains. My BRE-FRA flight was of course not a standalone flight but was a connection, the first leg of a long distance international journey. Otherwise I would not have flown it.