we did the train from krakow last time, and it was great with a buffet car that we sat in, reasonably cheap too, we are doing same route as you this time from Poznan on day of game, you can book the train on their website( you have to register first) from 30 days before, its about £40 return I think, their are 2 types of train EIC (express) about 2 and a half hours and TLK about 3hours 20 mins but much cheaper.
I am just waiting for the 30 day sale opening, once booked print off from home and thats yer ticket, On our return journey last time we had a carry out and no-one bothered. hope that helps