I just returned from a one-week trip to Poland. I highly recommend Poland to everyone.
We flew in and out of Warsaw and used the train to get to Karkow.
Lonely Planet has rules against promoting websites but I hope it is OK to share the one I used. I don't stand to gain anything by sharing the link. I used this site to buy my tickets and they are legitimate.
I booked 10 days before my trip and didn't want to chance the tickets not arriving by mail so I opted to have them left for me. The company sent me an email with a 1-page PDF document. The document had clear instructions in English regarding how to pick up my tickets. The tickets were left at a downtown post office that is open 24/7 so I was able to pick them up on a Sunday.
We rode a first class car on the 3-hour train. It was a 6-person compartment, comfortable enough for the trip but a little tight for luggage space. We booked out of the central train station in Warsaw and into the only train station in Krakow. The Karkow station (Krakow-Glowny?) is attached to the Krakow bus terminal.
Here's another good tip. You can take a train from the central warsaw transtation to the Chopin International Airport. We lucked out there, I was assuming we'd have to walk with our luggage to the subway or bus in Warsaw. However, we arrived in Warsaw at 12:47 PM and hopped on a 13:20 train to the airport. The airport run is 20 minutes or less.

