User blog comment:Yountoryuu/The Wikia's SBS/@comment-3177945-20110202235206/@comment-3326484-20110203004827

Supposedly there are only two ways to cross the red line. One is reverse mountain and thats a one way trip. The other is at Mariejois, where you can either go under, and through fishman island, or over, but it has not been revealed how they do that. As for Raftel, its generally assumed that getting there is harder than just sailing to the island, seeing as the Yonkou have been trying for several years with out success.