Aloha! That method of an interchange track works great, except... there is no limit on the interchange length, so the program keeps sending cars there regardless. The need on an interchange is a limit on # of cars or length (like a siding), so that if the interchange is full the program won't add any cars with that part of the route included. Actually, a yard should have a length or capacity setting anyway (although large for the off layout yards) since yards definitely have capacity limits...
Love the program otherwise,
Ben