Single exit rooms are not included as they only have one path anyway. When room exits are generated the game generates them in the order that the exit doors are internally numbered for the current room. Unfortunately we have no direct way of looking at which door is internally labeled as first. When exits in Tartarus are generated, the first exit generated must be the express route. In order to generate a non-express route exit, there must already be an express-route exit in the room, so non-express route exits must be generated second. This means that the express route door will always be whichever one Supergiant internally labeled as the "first door" of that room.