The rail load limit is the max bulk that can be contained on a rail. Attempts to address a carrier to a rail that has reached the limit will result in a Destination Full routing error. Mechanical routing can still send until the physical overload switch of the rail has been activated.
A normal empty carrier is bulk factor 1, but that can change depending on what article is loaded and what operations have been performed.
All carriers addressed to a rail or physically on a rail are counted as being part of the rail load.