It's possible to mount the trashcan mac pros with unofficial mounts, its just awkward and definitely not intended [0]. You can also mount iMac Pros [1].
[0] https://twitter.com/brianstucki/status/800778773998145536
It's also possible to outright buy Mac Pro Rack, a $6500 full depth 4U server, that is 1CPU all the way up, that offer neither any redundancy nor any remote management, if that kind of trash matters at all.
The lack of dual power supplies and remote power on/off and good PXE support is crazy for a rack mount computer (won’t even dignify it by calling it a server).
The rack mounted Mac Pro isn’t targeted at the server market, it’s targeted at creative industries (i.e, film/tv production, audio recording, etc) where you’d have it rackmounted into a flight case as part of a mobile post-production unit.
This has been an issue for decades. Twenty years ago I was doing grid/HPC computing at Sun and there were a lot of bioinformatics firms that used macs and wanted compute clusters of them. It was a huge problem even then.