Shenandoah River. To my knowledge, the other 3 River-series 10-6's listed earlier in this thread as being at TVRM are still there (if there's been news/status changes, I'm sure someone will reply here).

Update:

PRR-owned 10-6 sleeper Birch River (PRR #8351/PC #4356) from the Crescent pool was scrapped by the Railroaders Memorial Museum in Altoona over the course of October-November 2022: http://www.rypn.org/forums/viewtopic.php?f=1&t=47006