There are long distance trains like the RE (or ICE, which is not covered by this ticket) which are designed around everyone being seated, and there are commuter trains like the S-Bahn which are designed to have enough seating space for normal hours and lots of standing space for rush hour. The former are very uncomfortable when overfilled, the latter are designed around it in terms of ventilation, number of doors etc.
I've been in a couple of people-shoving-level crowded commuter trains when traveling to popular consumer trade fairs, and it was ok. Most people took it as part of the experience.
The biggest issue will be people trying to cross the country, packing 40 year old REs beyond their limit in the hottest time of year.
The more busy Sbahn lines (e.g. Ring Bahn) absolutely did get crowded enough that you couldnt even always get in pre-covid at peak times, and I suspect it will be even worse in the next 3 months.
I've been in a couple of people-shoving-level crowded commuter trains when traveling to popular consumer trade fairs, and it was ok. Most people took it as part of the experience.
The biggest issue will be people trying to cross the country, packing 40 year old REs beyond their limit in the hottest time of year.