Q: .migratedphotolibrary

Yes.

To be sure the story about aliasing the entries is not true, but the file in the Trash, but don't empty the Trash. Check that all your photos are in the Photos Library file before Emptying the Trash.

Yeah it appears that the image files are hard linked, so both files appear normal (and are actually basically identical) but there is only one file occupying space on disk.

So, can it be deleted?  What I've read indicates that both files are actually used...

I don't know. Read this before removing your migrated library. It seems that the size might be misleading and Photos still uses the old iPhoto library.

Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support

Photos creates hard links to the files in the iPhoto library, not symbolic links - and a hard link is very different from an alias or a symbolic link. If you create a hard link to a file, you can delete the original file without actually erasing the shared data.  The hard linked file will continue to work; as long as there is a hard link, the data will not be erased. Deletion of the blocks on the disk is based on reference counting for the hard links.

So you can either delete the original iPhoto Library or the migrated Photos library, and the shared image files will work in the remaining library.  Hard links are actual entries in the file table and not symbolic links to the name of another file.

The hard links between the images in the library are the same that Time Machine uses to avoid duplicate files on your backup drive.

Based on past experience you are right about being suspicious of linked files but the hard links are no problem.

iPhoto used symbolic links and not hard links, when you imported photos or videos without copying them to the iPhoto Library. For these photos with symbolic links it is essential to protect the reference files and not to delete them.

You can tell the difference between hard links and symbolic links or aliases in the Finder. A symbolic link or alias will show with an arrow on the icon, the hard linked files in the Masters folder look like any other file, without any arrow badge.

Have a look at this document: It explains more about the hard links used:  Six Colors: The (hard) link between Photos and iPhoto

But it is not necessary to delete the original iPhoto library, because the two libraries only take up the space of one library together. And you should keep the iPhoto library, until you are very sure that all photos migrated correctly and you have tested the new library and the Photos.app thoroughly. If you should discover missing photos or encounter an unexpected bug in Photos it will be very difficult to revert to iPhoto and to migrate the library again other wise.

Perfect, thats all I was looking to know. So keeping both for now or forever actually, doesn't matter at all.

I think (but would love for someone who KNOWS to confirm) that the one place where this will matter is when you start to delete your photos in an effort to clean house. If you delete photos from Photos and they are in your iPhoto bundle, then they will not actually be deleted unless you delete the photo from iPhoto as well.

So, the right way to migrate is really to trim down your iPhoto library as much as you plan to BEFORE migrating to Photos. Migrate to Photos. Keep the migratedphotolibrary file around for as long as you want.

Having gone about it the wrong way - which is to say I have a bunch of photos that I'd like to delete from Photos but don't want to have to delete also in iPhoto - I plan to copy the Photos Library.photoslibrary file to an external drive, delete both the migratedphotoslibrary and photoslibrary file on my internal hard drive, then copy the file back. That SHOULD (and again, would love confirmation - or will confirm once I back up and try it myself) eliminate hard links in lieu of actual files.

Well said, a hardlink to a file doesn't take the announced space on the disk, but that's only on that particular disk. Seen from the external point of view, there are two files, each one taking the full announced space on the disk, which means that when you copy these files to another volume, you do get two copies of the same original file, and these will indeed take double the room on that other volume.

So now if your online backup storage need for your photos suddenly doubles, you know why.