Its BUS till some changeover point (where they have managed in far too many yrs and for far too much money to rebuild it to normal gauge) AFAIK you do NOT need any RESer nor its even possible-the pass is your ticket, just like in any of EURs multitude of local/regional trains (also the 1 I now sit in; RE Hannover-Bremen, only 28 mins late.)
Do NOT use app for this planning-best is bah.com and this will clearly show train times AND also if RES is optional/mandatory or not even possible at all.
Its BUS till some changeover point (where they have managed in far too many yrs and for far too much money to rebuild it to normal gauge) AFAIK you do NOT need any RESer nor its even possible-
If you had checked, then you'd have seen that the OSE website offers reservation for the bus Patras-Kiato.