It would be a blessing to be able to lock in trains to specific routes that you want them to run indefinitly without them being changed when you hit "select all" when changing all your other engines.
Such as setting your bonus engines on a good and locking them in so that they are not affected when you change all the rest of your engines. I cant tell you how many times I've changed routes only to find out later that my kite got in the mix and its trying to go across country to the new destination at an agonizing slow rate. By the time you get to Lindworms, changing trains is a nightmare.