The Master.... says loading/unloading "can be" a preferred job.
"Part-time employees with six (6) months or more seniority shall have the right to place their name on the list of employees waiting to be moved to a preferred job within their building. Such preferred jobs shall include, but not be limited to: Preload, Sorter, Clerical, Irregular Train, Designated Responder, Carwasher, Loader and Unloader. Employees do not have the right to select any specific unit, load or workstation unless a prior past practice has been established."
I've never seen it posted as such, in my Local.
No one would sign it.