howdy, wasn't sure if it's intentional, there is no ability to stop a loader from putting items into a train without using external conditions on the station itself. it seems reasonable for a bulk rail (un)loader to have a valve of some kind that closes the chute for even loading/unloading, as seen in the real shipping fleets of the world.

From the description:
Sending a BRL the special "Disable rail loader" signal will stop the BRL from loading/unloading a cargo wagon. Due to their speed, you are likely to load or unload a few more items than expected.

oops. thanks.