Definitely walk. If you take the train, you get off in Toontown, and it doesn't open until ten, so if you get there at opening (as you should), you probably won't even be able to get off at Toontown. (Oops, just noticed NCDisneyMom said that already. Well, she's right.) Plus, Dumbo is right behind the castle, and that's where you're headed first. You ARE headed there first, right? Because that thing builds lines faster than anything in the park, and at five, it's a must-do.