I think it would be much easier to setup a train loading/unloading line if specific loaders/unloaders were turned ON or OFF with redstone signal. Or is there other way to enable/disable loader/unloader functionality? I think specifying carts is not enough, especially when it comes to item loading or unloading.
What others think about that?

Not technically possible in an intuitive way. You can use BC Gate Triggers to get some of this functionality.