Here's a KTX train ticket. It's about the size of a credit card.
I went to Busan last weekend to attend a family gathering, and it cost me a lot of money, but still, I could it was cheap when I consider the amount of time that was saved thanks to the fast train (Korea Train eXpress - KTX) .
If you're thinking of going from Seoul to Busan or the other way around, taking the KTX is the fastest, and you can buy a ticket very easily from any automatic ticket vending machine at Seoul Station (although Busan station doesn't have automatic ticket machines yet and you have to go to the ticket windows.)