Trains between Casa and Marrakech are reasonably fast and comfortable, and travel through beautiful scenery. First class cars are much preferable. It's possible that the ticket office would not sell tix for 1st class though, and instead recommend an "upgrade" on the train (thus allowing conductors to collect the difference). When you board, just tell the conductor that you're buying the upgrade, then sit anywhere you want in 1st class. The conductor will stop by later to collect the difference in fare (~$6).