I'm traveling from Copenhagen to Frankfurt and want to utilize an overnight train. I have a 3 Country Select Europass; 1) Do I have to reserve a seat for this particular train? 2) Where is the best place to find these tickets and choose my sleeping arrangements?
There are no longer any sleeping cars from Copenhagen to Frankfurt, just trains with normal seats where you have to change trains.
The brochure for German overnight trains is here: http://www.bahn.de/p_en/view/mdb/bahnintern/international/p_en/city_night_line/mdb_214178_final_db-cnl-flyer-jan2016-en_fuer_online.pdf
But most of these routes are closing at the end of this year.
For times and tickets for trains in, to or from Germany use the German Railways website: www.bahn.de (where the above brochure came from). Depending which train you pick, it will say if a reservation is required.
It appears there is still a night train from Berlin to Frankfurt, but Copenhagen to Berlin is half the trip, and the night train gets into Frankfurt at about 4 am, so why bother? Of course, if you are in a seat, it might not matter so much.
You can get from Copenhagen to Frankfurt on an ICE almost as fast as you can get from Copenhagen to Berlin.