Did a little railfanning of the DGNO around Denison, and spotted several cuts of gons and hoppers loaded with crushed rock. Among these cars were these two, obviously of MP heritage: Gondola with bzzsaw logo Hopper with "screaming eagle" logo
UP uses a lot of similar gons with UP, MP, SP and similar markings in rock train service out of Stringtown, OK (on the former Katy, now UP's Choctaw Sub) to Denison, where it is handed over to DGNO for shipment to either Royse City on the former MKT Greenville Sub or a spot on the former SP around Anna or Melissa.
Here's a train with some old MP hoppers at Moffat Tunnel, still in COAL serivce: YouTube - Moffat Tunnel: UP Coal Drag with 4 helpers! (MUST SEE) An old MP trailer I saw at UP's centennial yard, with original MPZ markings and a Buzzsaw logo! Even better: An old TPZ trailer!