The Pioneer was probably a short train because it was a part of The California Zephyr service (which also split off The Desert Wind to L.A. At one time, I believe all three services ran daily, but later there was some sort of alternate day service, with daily service only operating as far as Denver, where The Pioneer was cut off and sent through Wyoming to Portland and Seattle. I don't recall how the Oakland/San Francisco and Los Angeles services were divvied up.
My guess is that, unless Amtrak wants to invest in more Superliner equipment, which others have indicated is not in the works in the foreseeable future, a revival of either The Pioneer or The Desert Wind is unlikely. If the states of Oregon, Idaho, Utah, Nevada and California were willing (and able) to chip in to buy equipment, however, a more-or-less stand-alone service (i.e., not as part of the CZ) running from Portland (or even Seattle) to Salt Lake City, Las Vegas, and L.A. might be feasible. This would probably be run as one service between the Northwest and Salt Lake City, and another between Salt Lake, Vega$, and L.A. But the same vehicles could run all the way through, then be turned around at the other end.