If you book it online you don't need one (or at least I didn't for Hearts!)
Also, I with the "Not valid to Edinburgh" on the day we play Hibs away, it's perfectly valid to Inverkeithing, then pay the £2.60 to get a train (the same one you're already on) to Edinburgh. Also, if you book on the web-site, it doesn't seem to mind if you get a ticket straight through to Edinburgh on the 15th, and anyway, if a rugby train is anything like a football train for a Scotland game, there won't be any inspections on the train anyway.