We will be in Krakow for three days and then taking a night train from Krakow to Vienna in September. Any thoughts on purchasing tickets before via internet or can we wait and purchase at the train station in Krakow? We want to get a 2 person couchette or sleeper.
If it's an overnight train and you have to make reservations, I would do it ahead of time. Although it may be too early right now (not sure if Sept schedules are loaded in yet).
We purchased online tickets on an overnight train from Krakow to Prague this last September. I believe it could not be done earlier than 6 months in advance. But I was able to have an email sent to me when they were available at which time I did order them. It was an easy process and the trip worked out very well.
Two months in advance is the more likely date that booking opens, the old standard across Europe and still often the case the further east you go. Reserved tickets are sold by Polrail. You may have to use their "Custom Price Quote" function if the night train is not online. Station price is €103 per person in the double, per the Man in SEat 61. Waiting to buy tickets there would probably work if you were more flexible about the type of accommodation you wanted.