Probly there's a Strapac book that will tell more about the end of pooling, but IIRC there were short distance pools, like that on Tehachapi: LA-BAK or Colton-BAK.
that ended in the 70s, dominated by 6 axle UBoats. The SD-39s were also tailored to that work with low hp but ballasted for max adhesion and low speed TE.
Some nearly dedicated power served specific traffic after that, such as hi hp B-B power on Coast intermodals, concentration of GP-35s at Tucson, but changing power on long haul trains reduced availability and increased dwell time. Re-read Frailey's Blue Streak book to see some of the things that were done to speed it thru terminals. Shoot the EPBAA off the T&P would often go up the Coast with its UP engines- but maybe that was just SP stealing HP-hours