Ray

Question: How does Neal pay for the train tickets after they have just been robbed at the motel? Remember that Gus switched credit cards when they checked in. (00:35:50)

Answer: Gus only switched only one credit card. Neal has other credit cards. After the car fire, Neal tries to use several cards to pay for a hotel room. By then, the cards were all burned, but getting the train tickets was before the fire.

Ray

Answer: Neal didn't pay for them. Del mentioned a friend of his who works for the railroad owed him a favor so it's assumed the ride was complementary from Del's friend.

Del asks Neil for his address, so he can pay for the train ticket, Neil replies, "It's a gift" There was no mention of what favour Del was owed by his railroad friend.

Del asks Neal for his address specifically to pay for the train tickets.

Answer: Of course it's also possible that Del's friend guaranteed he'd make sure seats would be available but maybe still said they would still have to be purchased and NOT complimentary. It's a movie so any storyline here is a possibility.