As the topic is reflections on travel in Germany here are some of mine from the past 3 months:
1. International trains seem the least reliable: Amsterdam to Berlin was heavily delayed though not in Germany; Munich to Zurich was badly delayed by engineering works which meant it lost paths.
2. IC 2 trains seem to have more space on the lower deck generally and lower deck can be a lot cooler on warm days.
3. Diesel regional services seem pretty reliable, several I travelled on left late awaiting connections but made up time en route.
4. The old East Germany is still a maze of disused lines, grassy sidings etc. From Frankfurt am Oder down to Zittau it's pretty but you are never far from signs of obsolete industry (think South Yorkshire).
5. Steam is well worth seeking out, many preserved narrow gauge lines, the one from Zittau makes an easy half day outing.
6. Long distance IC trains can be quieter than ICE, maybe because they are slower? But also have comfy seats and longer trains means there's somewhere to wander if your legs get stiff on a long journey.
7. S bahn in Berlin has no toilets on board!
8. If Rhine valley trains packed out, try the opposite bank line from Wiesbaden to Koblenz, the scenery is just as good.
9. From Bonn there's a good alternative though slow route to Koln via Koln Flughafen. Whenever I've used this it seems half empty.
10. Ditto Koln to Dusseldorf can be a squeeze on local trains. Going via Monchengladbach is slow but quieter.
Generally stations and trains are cleaner than UK, but every station has its fair share of smokers, and this is pretty standard across Europe.
I managed to get quite a lot of loco haulage by referring to Wagonweb and other forums, but most regional trains were DMU or Emu. Standards on these varied between cramped to spacious, older stock has better window/seat alignment I found.
Interrail is great no questions asked anywhere, but I'd recommend reserving seats on long distance trains using Bahn.de website to avoid shunting round on ICEs, ICs always seemed to have space somewhere.
Last summer I tried the 9 euro ticket and virtually every train was wedged. Certainly not quite as crazily overcrowded this summer....so far.
It's worth finding out when school holidays are. Actually June is busy with school trips by train. Beware, that quiet 2 car dmu can suddenly be inundated with 25 small children and their teachers!
I found travelling round Germany by train to be easy, people were generally friendly and many spoke perfect English. Many on board announcements are in English too, particularly on ICE.