In both cases [a] or [b], it makes perfect sense for the handling of "foreign" and "home" rolling stock based on our previous thread concerning the expansive movement of "foreign" cars. They arrive from and they exit to hidden staging. The engine is irrelevent.
Where I need some help is to better understand freight engine movement.
I sometimes see model RR images of mixed, freight consists ["foreign" and "home" engines], and I wonder if that can only happen with option [b]. It would make sense that two railroads who require one another's assistance have a contractual agreement to operate mixed consists. Those lash ups can occur at whichever on-line interchange yard directly interacts with hidden staging.
Without option [b], I cannot see any justification for an all "home", freight engine consist exiting to hidden staging or arriving from hidden staging.
Thanks! Matt