Guides
The money-and-sanity-saving stuff, written to be the best answer on the internet.
Rail
Seat reservations: the hidden cost of European rail
Compulsory-reservation trains cost €10–35 extra per leg on top of a pass, every leg.
Rail passes vs point-to-point
Price every leg as an advance fare first, then compare against pass plus compulsory reservations.
Night trains in Europe: an honest appraisal
The couchette-to-sleeper price gap is the whole decision; price both before you romanticise either.