I had the same problem and I discovered that the overlays scope (for the *_Vendor and *_Local files) are in the current directory. If you put an loca/lib/RT/Queue_Local.pm, you’re going to “overwrite” the lib/RT/Queue_Local.pm: you’re not doing the overlay for Queue.pm.
So, you need to copy the lib/RT/Queue.pm to local/lib/RT/Queue.pm, and only after the local copy you develop and see the effect of your overlay. If you want, you could remove all the code from the local/lib/Queue.pm, but it’s necessary to keep the final lines of the file, that activate the File_Vender or File_Local if it exists.
Lucas F. ROsada