Unfortunately, it seems that the EN446 'Jan Kiepura' nighttrain Warsaw-Poznan-Berlin-Cologne-Amsterdam is being discontinued from end of June:
The end of the Jan Kiepura from 1 July...
It now looks almost certain that the Amsterdam/Cologne to Warsaw Jan Kiepura will be discontinued from 1 July 2013, the last in a series of cuts by Polish Railways. ... The Jan Kipeura's last westbound departure from Warsaw will be 29 June, it's last eastbound from Amsterdam will be 30 June.
I haven't seen any confirmation (in English) of this on the PKP site, but the train no longer exists in the
bahn.de journey planner.
The only direct overnight connection between Poznan and Frankfurt(main) currently is the RZD through sleeper carriage Moscow-Minsk-Warsaw-Berlin-Frankfurt-Basel - and as this runs with the 'Jan Kiepura' between Warsaw and Berlin, it seems as if it will be discontinued as well. (Although there's currently an EC 'Berlin-Warsaw Express' that runs about 40 minutes ahead of the Jan Kiepura, so perhaps RZD will work out some way to attach their carriages to that - although I'm not too optimistic...)
All very unfortunate, as the Jan Kiepura and associated through carriages provided a very useful link between 'western' and 'eastern' Europe. I understand it was quite well used, and the other services it ran in conjunction with are still running, so I'm not sure why it has been cancelled...