Hi Szymon,
the most of german cycling ways on former railway track you will find here:
https://de.wikipedia.org/wiki/Liste_von_Fahrradwegen_auf_stillgelegten_Bahntrassen#Baden-Württemberg.
If you in May in the south on the Mainradweg you could try to come to our Meeting:
Forumstreffen Himmelfahrt 2018 in Lauterbourg (Treffpunkt)There will be 100 of us and you need only a tent and a bycicle.
Between Frankfurt an Berlin it is a little bit boring. Like Wielkopolksa.
You can use these way or some parts of it:
http://www.thueringer-staedtekette.de/index.php?id=42&l=1Gruß Uwe