At the end of Oklahoma!, Curly and Laurey get married and the characters literally sing the praises of their soon-to-be state, declaring it to be a place of natural beauty, agricultural plenty, and boundless promise. Which becomes very ironic when you realize that the Dust Bowl will happen within the happy couple's lifetime.
Not to mention the absolutely brutal weather that the state experiences on a yearly basis. If Oklahoma is famous for one thing in particular, it's tornados.
Curly hired the surrey for the Box Social. The wedding happens weeks later according to the dialogue. So why is it available again for the closing scene where the suggestion is made to take the couple down to the train in "Curly's Surry"?