This may be a bit "left field" but what about using a system that doesn't require the party to always be together (or even * gasp * be cooperative!) I've been playing Apocalypse World quite a bit lately, and we've had some very interesting PC vs PC conflict. Admittedly the game is a lot more story driven than D&D, and if you are really interested in the Fantasy setting then Dungeon World might be more appropriate. Regardless, if you are dead set on not railroading, blowing up the whole damn train might be a good start. There are other systems out there as well that don't rely "Don't Split the Party."

This may be a bit "left field" but what about using a system that doesn't require the party to always be together (or even *gasp* be cooperative!) I've been playing Apocalypse World quite a bit lately, and we've had some very interesting PC vs PC conflict. Admittedly the game is a lot more story driven than D&D, and if you are really interested in the Fantasy setting then Dungeon World might be more appropriate. Regardless, if you are dead set on not railroading, blowing up the whole damn train might be a good start. There are other systems out there as well that don't rely on "Don't Split the Party."