I think they don't consider that because it's a rail system. Re-arranging cars on route would require many junctions, and would probably slow down the whole system when they separate. It's also a different problem that they are solving with warehouse robots - goods need to be transported individually to destinations. Trains solve mass transport. And besides - autonomous rail robots would still be bound to a rail, and thus can't really plan any fancy avoidance or path finding that a warehouse robot could.